In the coming years, many ground-based and space-based telescopes will commence operations and collect their first light from cosmic sources. This next-generation of telescopes is not only expected to see farther into the cosmos (and hence, farther back in time), they are also expected to reveal new things about the nature of our Universe, its creation and its evolution.
One of these instruments is the Extremely Large Telescope, an optical telescope that is overseen by the European Southern Observatory. Once it is built, the ELT will be the largest ground-based telescope in the world. Construction began in May of 2017, and the ESO recently released a video that illustrates what it will look like when it is complete.
In the 1920s, Edwin Hubble made the groundbreaking discovery that the Universe was in a state of expansion. Originally predicted as a consequence of Einstein’s Theory of General Relativity, measurements of this expansion came to be known as Hubble’s Constant. Today, and with the help of next-generation telescopes – like the aptly-named Hubble Space Telescope (HST) – astronomers have remeasured and revised this law many times.
These measurements confirmed that the rate of expansion has increased over time, though scientists are still unsure why. The latest measurements were conducted by an international team using Hubble, who then compared their results with data obtained by the European Space Agency’s (ESA) Gaia observatory. This has led to the most precise measurements of the Hubble Constant to date, though questions about cosmic acceleration remain.
Since 2005, Adam Riess – a Nobel Laureate Professor with the Space Telescope Science Institute and the Johns Hopkins University – has been working to refine the Hubble Constant value by streamlining and strengthening the “cosmic distance ladder”. Along with his team, known as Supernova H0 for the Equation of State (SH0ES), they have successfully reduced the uncertainty associated with the rate of cosmic expansion to just 2.2%
To break it down, astronomers have traditionally used the “cosmic distance ladder” to measure distances in the Universe. This consists of relying on distance markers like Cepheid variables in distant galaxies – pulsating stars whose distances can be inferred by comparing their intrinsic brightness with their apparent brightness. These measurements are then compared to the way light from distant galaxies is redshifted to determine how fast the space between galaxies is expanding.
From this, the Hubble Constant is derived. Another method that is used is to observe the Cosmic Microwave Background (CMB) to trace the expansion of the cosmos during the early Universe – circa. 378,000 years after the Big Bang – and then using physics to extrapolate that to the present expansion rate. Together, the measurements should provide an end-to-end measurement of how the Universe has expanded over time.
However, astronomers have known for some time that the two measurements don’t match up. In a previous study, Riess and his team conducted measurements using Hubble to obtain a Hubble Constant value of 73 km/s (45.36 mps) per megaparsec (3.3 million light-years). Meanwhile, results based on the ESA’ Planck observatory (which observed the CMB between 2009 and 2013) predicted that the Hubble constant value should now be 67 km/s (41.63 mps) per megaparsec and no higher than 69 km/s (42.87 mps) – which represents a discrepancy of 9%.
“The tension seems to have grown into a full-blown incompatibility between our views of the early and late time universe. At this point, clearly it’s not simply some gross error in any one measurement. It’s as though you predicted how tall a child would become from a growth chart and then found the adult he or she became greatly exceeded the prediction. We are very perplexed.”
In this case, Riess and his colleagues used Hubble to gauge the brightness of distant Cepheid variables while Gaia provided the parallax information – the apparent change in an objects position based on different points of view – needed to determine the distance. Gaia also added to the study by measuring the distance to 50 Cepheid variables in the Milky Way, which were combined with brightness measurements from Hubble.
This allowed the astronomers to more accurately calibrate the Cepheids and then use those seen outside the Milky Way as milepost markers. Using both the Hubble measurements and newly released data from Gaia, Riess and his colleagues were able to refine their measurements on the present rate of expansion to 73.5 kilometers (45.6 miles) per second per megaparsec.
As Stefano Casertano, of the Space Telescope Science Institute and a member of the SHOES team, added:
“Hubble is really amazing as a general-purpose observatory, but Gaia is the new gold standard for calibrating distance. It is purpose-built for measuring parallax—this is what it was designed to do. Gaia brings a new ability to recalibrate all past distance measures, and it seems to confirm our previous work. We get the same answer for the Hubble constant if we replace all previous calibrations of the distance ladder with just the Gaia parallaxes. It’s a crosscheck between two very powerful and precise observatories.”
Looking to the future, Riess and his team hope to continue to work with Gaia so they can reduce the uncertainty associated with the value of the Hubble Constant to just 1% by the early 2020s. In the meantime, the discrepancy between modern rates of expansion and those based on the CMB will continue to be a puzzle to astronomers.
In the end, this may be an indication that other physics are at work in our Universe, that dark matter interacts with normal matter in a way that is different than what scientists suspect, or that dark energy could be even more exotic than previously thought. Whatever the cause, it is clear the Universe still has some surprises in store for us!
Since the 1990s, astrophysicists have known that for the past few billion years, the Universe has been experiencing an accelerated rate of expansion. This gave rise to the theory that the Universe is permeated by a mysterious invisible energy known as “dark energy”, which acts against gravity and is pushing the cosmos apart. In time, this energy will become the dominant force in the Universe, causing all stars and galaxies to spread beyond the cosmic horizon.
At this point, all stars and galaxies in the Universe will no longer be visible or accessible from any other. The question remains, what will intelligent civilizations (such as our own) do for resources and energy at this point? This question was addressed in a recent paper by Dr. Abraham Loeb – the Frank B. Baird, Jr., Professor of Science at Harvard University and the Chair of the Harvard Astronomy Department.
The paper, “Securing Fuel for our Frigid Cosmic Future“, recently appeared online. As he indicates in his study, when the Universe is ten times its current age (roughly 138 billion years old), all stars outside the Local Group of galaxies will no be accessible to us since they will be receding away faster than the speed of light. For this reason, he recommends that humanity follow the lesson from Aesop’s fable, “The Ants and the Grasshopper”.
This classic tale tells the story of ants who spent the summer collecting food for the winter while the grasshopper chose to enjoy himself. While different versions of the story exist that offer different takes on the importance of hard work, charity, and compassion, the lesson is simple: always be prepared. In this respect, Loeb recommends that advanced species migrate to rich clusters of galaxies.
These clusters represent the largest reservoirs of matter bound by gravity and would therefore be better able to resist the accelerated expansion of the Universe. As Dr. Loeb told Universe Today via email:
“In my essay I point out that mother Nature was kind to us as it spontaneously gave birth to the same massive reservoir of fuel that we would have aspired to collect by artificial means. Primordial density perturbations from the early universe led to the gravitational collapse of regions as large as tens of millions of light years, assembling all the matter in them into clusters of galaxies – each containing the equivalent of a thousand Milky Way galaxies.”
Dr. Loeb also indicated where humanity (or other advanced civilizations) should consider relocating to when the expansion of the Universe causes the stars of the Local Group to expand beyond the cosmic horizon. Within 50 million light years, he indicates, likes the Virgo Cluster, which contains about a thousands times more matter than the Milky Way Galaxy. The second closest is the Coma Cluster, a collection of over 1000 galaxies located about 336 million light years away.
In addition to offering a solution to the accelerating expansion of the Universe, Dr. Loeb’s study also presents some interesting possibilities when it comes to the search for extra-terrestrial intelligence (SETI). If, in fact, there are already advanced civilizations migrating to prepare for the inevitable expansion of the Universe, they may be detectable by various means. As Dr. Loeb explained:
“If traveling civilizations transmit powerful signals then we might be able to see evidence for their migration towards clusters of galaxies. Moreover, we would expected a larger concentration of advanced civilization in clusters than would be expected simply by counting the number of galaxies there. Those that settle there could establish more prosperous communities, in analogy to civilizations near rivers or lakes on Earth.”
This paper is similar to a study Dr. Loeb conducted back in 2011, which appeared in the Journal of Cosmology and Astroparticle Physics under the title “Cosmology with Hypervelocity Stars“. At the time, Dr. Loeb was addressing what would happen in the distant future when all extragalactic light sources will cease to be visible or accessible due to the accelerating expansion of the Universe.
This study was a follow-up to a 2001 paper in which Dr. Loeb addressed what would become of the Universe in billions of years – which appeared in the journal Physical Review Letters under the title “The Long–Term Future of Extragalactic Astronomy“. Shortly thereafter, Dr. Loeb and Freeman Dyson himself began to correspond about what could be done to address this problem.
Their correspondence was the subject of an article by Nathan Sanders (a writer for Astrobites) who recounted what Dr. Loeb and Dr. Dyson had to say on the matter. As Dr. Loeb recalls:
“A decade ago I wrote a few papers on the long-term future of the Universe, trillions of years from now. Since the cosmic expansion is accelerating, I showed that once the universe will age by a factor of ten (about a hundred billion years from now), all matter outside our Local Group of galaxies (which includes the Milky Way and the Andromeda galaxy, along with their satellites) will be receding away from us faster than light. After one of my papers was posted in 2011, Freeman Dyson wrote to me and suggested to a vast “cosmic engineering project” in which we will concentrate matter from a large-scale region around us to a small enough volume such that it will stay bound by its own gravity and not expand with the rest of the Universe.”
At the time, Dr. Loeb indicated that data gathered by the Sloan Digital Sky Survey (SDSS) indicated that attempts at “super-engineering” did not appear to be taking place. This was based on the fact that the galaxy clusters observed by the SDSS were not overdense, nor did they exhibit particularly high velocities (as would be expected). To this, Dr. Dyson wrote: “That is disappointing. On the other hand, if our colleagues have been too lazy to do the job, we have plenty of time to start doing it ourselves.”
A similar idea was presented in a recent paper by Dr. Dan Hooper, an astrophysicist from the Fermi National Accelerator Laboratory (FNAL) and the University of Chicago. In his study, Dr. Hooper suggested that advanced species could survive all stars in the Local Group expanding beyond the cosmic horizon (100 billion years from now), by harvesting stars across tens of millions of light years.
This harvesting would consist of building unconventional Dyson Spheres that would use the energy they collected from stars to propel them towards the center of the species’ civilization. However, only stars that range in mass of 0.2 to 1 Solar Masses would be usable, as high-mass stars would evolve beyond their main sequence before reaching the destination and low-mass stars would not generate enough energy for acceleration to make it in time.
But as Dr. Loeb indicates, there are additional limitations to this approach, which makes migrating more attractive than harvesting.
“First, we do not know of any technology that enables moving stars around, and moreover Sun-like stars only shine for about ten billion years (of order the current age of the Universe) and cannot serve as nuclear furnaces that would keep us warm into the very distant future. Therefore, an advanced civilization does not need to embark on a giant construction project as suggested by Dyson and Hooper, but only needs to propel itself towards the nearest galaxy cluster and take advantage of the cluster resources as fuel for its future prosperity.”
While this may seem like a truly far-off concern, it does raise some interesting questions about the long-term evolution of the Universe and how intelligent civilizations may be forced to adapt. In the meantime, if it offers some additional possibilities for searching for extra-terrestrial intelligences (ETIs), then so much the better.
And as Dr. Dyson said, if there are currently no ETIs preparing for the coming “cosmic winter” with cosmic engineering projects, perhaps it is something humanity can plan to tackle someday!
During the 1930s, astronomers came to realize that the Universe is in a state of expansion. By the 1990s, they realized that the rate at which it is expansion is accelerating, giving rise to the theory of “Dark Energy”. Because of this, it is estimated that in the next 100 billion years, all stars within the Local Group – the part of the Universe that includes a total of 54 galaxies, including the Milky Way – will expand beyond the cosmic horizon.
At this point, these stars will no longer be observable, but inaccessible – meaning that no advanced civilization will be able to harness their energy. Addressing this, Dr. Dan Hooper – an astrophysicist from the Fermi National Accelerator Laboratory (FNAL) and the University of Chicago – recently conducted a study that indicated how a sufficiently advanced civilization might be able to harvest these stars and prevent them from expanding outward.
To put it simply, the theory of Dark Energy is that space is filled with a mysterious invisible force that counteracts gravity and causes the Universe to expand at an accelerating rate. The theory originated with Einstein’s Cosmological Constant, a term he added to his theory of General Relativity to explain how the Universe could remain static, rather than be in a state of expansion or contraction.
While Einstein was proven wrong, thanks to observations that showed that the Universe was expanding, scientists revisited the concept in order to explain how cosmic expansion has sped up in the past few billion years. The only problem with this theory, according to Dr. Hooper’s study, is that the dark energy will eventually become dominant, and the rate of cosmic expansion Universe will increase exponentially.
As a result, the Universe will expand to the point where all stars are so far apart that intelligent species won’t even be able to see them, let alone explore them or harness their energy. As Dr. Hooper told Universe Today via email:
“Cosmologists have learned over the last 20 years that our universe is expanding at an accelerating rate. This means that over the next 100 billion years or so, most of the stars and galaxies that we can now see in the sky will disappear forever, falling beyond any regions of space that we could reach, even in principle. This will limit the ability of a far-future advanced civilization to collect energy, and thus limit any number of things they might want to accomplish.”
In addition to being the Head of the Theoretical Astrophysics Group at the FNAL, Dr. Hooper is also an Associate Professor in the Department of Astronomy and Astrophysics at the University of Chicago. As such, he is well versed when it comes to the big questions of extra-terrestrial intelligence (ETI) and how cosmic evolution will affect intelligent species.
To tackle how advanced civilizations would go about living in such a Universe, Dr. Hooper begins by assuming that the civilizations in question would be a Type III on the Kardashev scale. Named in honor of Russian astrophysicist Nikolai Kardashev, a Type III civilization would have reached galactic proportions and could control energy on a galactic scale. As Hooper indicated:
“In my paper, I suggest that the rational reaction to this problem would be for the civilization to expand outward rapidly, capturing stars and transporting them to the central civilization, where they could be put to use. These stars could be transported using the energy they produce themselves.”
As Dr. Hooper admits, this conclusion relies on two assumptions – first, that a highly advanced civilization will attempt to maximize its access to usable energy; and second, that our current understanding of dark energy and the future expansion of our Universe is approximately correct. With this in mind, Dr. Hooper attempted to calculate which stars could be harvested using Dyson Spheres and other megastructures.
This harvesting, according to Dr. Hooper, would consist of building unconventional Dyson Spheres that would use the energy they collected from stars to propel them towards the center of the species’ civilization. High-mass stars are likely to evolve beyond the main sequence before reaching the destination of the central civilization and low-mass stars would not generate enough energy (and therefore acceleration) to avoid falling beyond the horizon.
For these reasons, Dr. Hooper concludes that stars with masses of between 0.2 and 1 Solar Masses will be the most attractive targets for harvesting. In other words, stars that are like our Sun (G-type, or yellow dwarf), orange dwarfs (K-type), and some M-type (red dwarf) stars would all be suitable for a Type III civilization’s purposes. As Dr. Hooper indicates, there would be limiting factors that have to be considered:
“Very small stars often do not produce enough energy to get them back to the central civilization. On the other hand, very large stars are short lived and will run out of nuclear fuel before they reach their destination. Thus the best targets of this kind of program would be stars similar in size (or a little smaller) than the Sun.”
Based on the assumption that such a civilization could travel at 1 – 10% the speed of light, Dr. Hooper estimates that they would be able to harvest stars out to a co-moving radius of approximately 20 to 50 Megaparsecs (about 65.2 million to 163 million light-years). Depending on their age, 1 to 5 billion years, they would be able to harvest stars within a range of 1 to 4 Megaparsecs (3,260 to 13,046 light-years) or up to several tens of Megaparsecs.
In addition to providing a framework for how a sufficiently-advanced civilization could survive cosmic acceleration, Dr. Hooper’s paper also provides new possibilities in the search for extra-terrestrial intelligence (SETI). While his study primarily addresses the possibility that such a mega-civilization will emerge in the future (perhaps it will even be our own), he also acknowledges the possibility that one could already exist.
In the past, scientists have suggested looking for Dyson Spheres and other megastructures in the Universe by looking for signatures in the infrared or sub-millimeter bands. However, megastructures that have been built to completely harvest the energy of a star, and use it to transport them across space at relativistic speeds, would emit entirely different signatures.
In addition, the presence of such a mega-civilization could be discerned by looking at other galaxies and regions of space to see if a harvesting and transport process has already begun (or is in an advanced stage). Whereas past searchers for Dyson Spheres have focused on detecting the presence of structures around individual stars within the Milky Way, this kind of search would focus on galaxies or groups of galaxies in which most of the stars would be surrounded by Dyson Spheres and removed.
“This provides us with a very different signal to look for,” said Dr. Hooper. “An advanced civilization that is in the process of this program would alter the distribution of stars over regions of space tens of millions of light years in extent, and would likely produce other signals as a result of stellar propulsion.”
In the end, this theory not only provides a possible solution for how advanced species might survive cosmic expansion, it also offers new possibilities in the hunt for extra-terrestrial intelligence. With next-generation instruments looking farther into the Universe and with greater resolution, perhaps we should be on the lookout for hypervelocity stars that are all being transported to the same region of space.
Could be a Type III civilization preparing for the day when dark energy takes over!
The Multiverse Theory, which states that there may be multiple or even an infinite number of Universes, is a time-honored concept in cosmology and theoretical physics. While the term goes back to the late 19th century, the scientific basis of this theory arose from quantum physics and the study of cosmological forces like black holes, singularities, and problems arising out of the Big Bang Theory.
One of the most burning questions when it comes to this theory is whether or not life could exist in multiple Universes. If indeed the laws of physics change from one Universe to the next, what could this mean for life itself? According to a new series of studies by a team of international researchers, it is possible that life could be common throughout the Multiverse (if it actually exists).
Together, the research team sought to determine how the accelerated expansion of the cosmos could have effected the rate of star and galaxy formation in our Universe. This accelerate rate of expansion, which is an integral part of the Lambda-Cold Dark Matter (Lambda-CDM) model of cosmology, arose out of problems posed by Einstein’s Theory of General Relativity.
As a consequence of Einstein’s field equations, physicist’s understood that the Universe would either be in a state of expansion or contraction since the Big Bang. In 1919, Einstein responded by proposing the “Cosmological Constant” (represented by Lambda), which was a force that “held back” the effects of gravity and thus ensured that the Universe was static and unchanging.
Shortly thereafter, Einstein retracted this proposal when Edwin Hubble revealed (based on redshift measurements of other galaxies) that the Universe was indeed in a state of expansion. Einstein apparently went as far as to declare the Cosmological Constant “the biggest blunder” of his career as a result. However, research into cosmological expansion during the late 1990s caused his theory to be reevaluated.
In short, ongoing studies of the large-scale Universe revealed that during the past 5 billion years, cosmic expansion has accelerated. As such, astronomers began to hypothesize the existence of a mysterious, invisible force that was driving this acceleration. Popularly known as “Dark Energy”, this force is also referred to as the Cosmological Constant (CC) since it is responsible for counter-effecting the effects of gravity.
Since that time, astrophysicists and cosmologists have sought to understand how Dark Energy could have effected cosmic evolution. This is an issue since our current cosmological models predict that there must be more Dark Energy in our Universe than has been observed. However, accounting for larger amounts of Dark Energy would cause such a rapid expansion that it would dilute matter before any stars, planets or life could form.
For the first study, Salcido and the team therefore sought to determine how the presence of more Dark Energy could effect the rate of star formation in our Universe. To do this, they conducted hydrodynamical simulations using the EAGLE (Evolution and Assembly of GaLaxies and their Environments) project – one of the most realistic simulations of the observed Universe.
Using these simulations, the team considered the effects that Dark Energy (at its observed value) would have on star formation over the past 13.8 billion years, and an additional 13.8 billion years into the future. From this, the team developed a simple analytic model that indicated that Dark Energy – despite the difference in the rate of cosmic expansion – would have a negligible impact on star formation in the Universe.
They further showed that the impact of Lambda only becomes significant when the Universe has already produced most of its stellar mass and only causes decreases in the total density of star formation by about 15%. As Salcido explained in a Durham University press release:
“For many physicists, the unexplained but seemingly special amount of dark energy in our Universe is a frustrating puzzle. Our simulations show that even if there was much more dark energy or even very little in the Universe then it would only have a minimal effect on star and planet formation, raising the prospect that life could exist throughout the Multiverse.”
For the second study, the team used the same simulation from the EAGLE collaboration to investigate the effect of varying degrees of the CC on the formation on galaxies and stars. This consisted of simulating Universes that had Lambda values ranging from 0 to 300 times the current value observed in our Universe.
However, since the Universe’s rate of star formation peaked at around 3.5 billion years before the onset of accelerating expansion (ca. 8.5 billion years ago and 5.3 billion years after the Big Bang), increases in the CC had only a small effect on the rate of star formation.
Taken together, these simulations indicated that in a Multiverse, where the laws of physics may differ widely, the effects of more dark energy cosmic accelerated expansion would not have a significant impact on the rates of star or galaxy formation. This, in turn, indicates that other Universes in the Multiverse would be just about as habitable as our own, at least in theory. As Dr. Barnes explained:
“The Multiverse was previously thought to explain the observed value of dark energy as a lottery – we have a lucky ticket and live in the Universe that forms beautiful galaxies which permit life as we know it. Our work shows that our ticket seems a little too lucky, so to speak. It’s more special than it needs to be for life. This is a problem for the Multiverse; a puzzle remains.”
However, the team’s studies also cast doubt on the ability of Multiverse Theory to explain the observed value of Dark Energy in our Universe. According to their research, if we do live in a Multiverse, we would be observing as much as 50 times more Dark Energy than what we are. Although their results do not rule out the possibility of the Multiverse, the tiny amount of Dark Energy we’ve observed would be better explained by the presence of a as-yet undiscovered law of nature.
As Professor Richard Bower, a member of Durham University’s Institute for Computational Cosmology and a co-author on the paper, explained:
“The formation of stars in a universe is a battle between the attraction of gravity, and the repulsion of dark energy. We have found in our simulations that Universes with much more dark energy than ours can happily form stars. So why such a paltry amount of dark energy in our Universe? I think we should be looking for a new law of physics to explain this strange property of our Universe, and the Multiverse theory does little to rescue physicists’ discomfort.”
These studies are timely since they come on the heels of Stephen Hawking’s final theory, which cast doubt on the existence of the Multiverse and proposed a finite and reasonably smooth Universe instead. Basically, all three studies indicate that the debate about whether or not we live in a Multiverse and the role of Dark Energy in cosmic evolution is far from over. But we can look forward to next-generation missions providing some helpful clues in the future.
What’s more, all of these missions are expected to be gathering their first light sometime in the 2020s. So stay tuned, because more information – with cosmological implications – will be arriving in just a few years time!
The updates continue. Last week we talked about dark matter, and this week we continue with its partner dark energy. Of course, they’re not really partners, unless you consider mysteriousness to be an attribute. Dark energy, that force that’s accelerating the expansion of the Universe. What have we learned?
A supernova is one of the most impressive natural phenomena in the Universe. Unfortunately, such events are often brief and transient, temporarily becoming as bright as an entire galaxy and then fading away. But given what these bright explosions – which occur when a star reaches the end of its life cycle – can teach us about the Universe, scientists are naturally very interested in studying them.
The team was led by Miika Pursiainen, a PhD researcher from the University of Southampton. For the sake of their study, the team relied on data from the 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO). This telescope is part of the Dark Energy Survey, a global effort to map hundreds of millions of galaxies and thousands of supernovae in to find patterns int he cosmic structure that will reveal the nature of dark energy.
As Pursiainen commented in a recent Southampton news release:
“The DES-SN survey is there to help us understand dark energy, itself entirely unexplained. That survey then also reveals many more unexplained transients than seen before. If nothing else, our work confirms that astrophysics and cosmology are still sciences with a lot of unanswered questions!”
As noted, these events were very peculiar in that they had a similar maximum brightness compared to different types of supernove, they were visible for far less time. Whereas supernova typically last for several months or more, these transient supernovae were visible for about a week to a month. The events also appeared to be very hot, with temperatures ranging from 10,000 to 30,000 °C (18,000 to 54,000 °F).
They also vary considerably in size, ranging from being several times the distance between the Earth and the Sun – 150 million km, 93 million mi (or 1 AU) – to hundreds of times. However, they also appear to be expanding and cooling over time, which is what is expected from an event like a supernova. Because of this, there is much debate about the origin of these transient supernovae.
A possible explanation is that these stars shed a lot of material before they exploded, and that this could have shrouded them in matter. This material may then have been heated by the supernovae themselves, causing it to rise to very high temperatures. This would mean that in these cases, the team was seeing the hot clouds rather than the exploding stars themselves.
This certainly would explain the observations made by Pursiainen and his team, though a lot more data will be needed to confirm this. In the future, the team hopes to examine more transients and see how often they occur compared to more common supernovae. The study of this powerful and mysterious phenomenon will also benefit from the use of next-generation telescopes.
When the James Webb Space Telescope is deployed in 2020, it will study the most distant supernovae in the Universe. This information, as well as studies performed by ground-based observatories, is expected to not only shed light on the life cycle of stars and dark energy, but also on the formation of black holes and gravitational waves.
In the 1920s, Edwin Hubble made the groundbreaking revelation that the Universe was in a state of expansion. Originally predicted as a consequence of Einstein’s Theory of General Relativity, this confirmation led to what came to be known as Hubble’s Constant. In the ensuring decades, and thanks to the deployment of next-generation telescopes – like the aptly-named Hubble Space Telescope (HST) – scientists have been forced to revise this law.
In short, in the past few decades, the ability to see farther into space (and deeper into time) has allowed astronomers to make more accurate measurements about how rapidly the early Universe expanded. And thanks to a new survey performed using Hubble, an international team of astronomers has been able to conduct the most precise measurements of the expansion rate of the Universe to date.
This tool is how astronomers have traditionally measured distances in the Universe, which consists of relying on distance markers like Cepheid variables – pulsating stars whose distances can be inferred by comparing their intrinsic brightness with their apparent brightness. These measurements are then compared to the way light from distance galaxies is redshifted to determine how fast the space between galaxies is expanding.
From this, the Hubble Constant is derived. To build their distant ladder, Riess and his team conducted parallax measurements using Hubble’s Wide Field Camera 3 (WFC3) of eight newly-analyzed Cepheid variable stars in the Milky Way. These stars are about 10 times farther away than any studied previously – between 6,000 and 12,000 light-year from Earth – and pulsate at longer intervals.
To ensure accuracy that would account for the wobbles of these stars, the team also developed a new method where Hubble would measure a star’s position a thousand times a minute every six months for four years. The team then compared the brightness of these eight stars with more distant Cepheids to ensure that they could calculate the distances to other galaxies with more precision.
Using the new technique, Hubble was able to capture the change in position of these stars relative to others, which simplified things immensely. As Riess explained in a NASA press release:
“This method allows for repeated opportunities to measure the extremely tiny displacements due to parallax. You’re measuring the separation between two stars, not just in one place on the camera, but over and over thousands of times, reducing the errors in measurement.”
Compared to previous surveys, the team was able to extend the number of stars analyzed to distances up to 10 times farther. However, their results also contradicted those obtained by the European Space Agency’s (ESA) Planck satellite, which has been measuring the Cosmic Microwave Background (CMB) – the leftover radiation created by the Big Bang – since it was deployed in 2009.
By mapping the CMB, Planck has been able to trace the expansion of the cosmos during the early Universe – circa. 378,000 years after the Big Bang. Planck’s result predicted that the Hubble constant value should now be 67 kilometers per second per megaparsec (3.3 million light-years), and could be no higher than 69 kilometers per second per megaparsec.
Based on their sruvey, Riess’s team obtained a value of 73 kilometers per second per megaparsec, a discrepancy of 9%. Essentially, their results indicate that galaxies are moving at a faster rate than that implied by observations of the early Universe. Because the Hubble data was so precise, astronomers cannot dismiss the gap between the two results as errors in any single measurement or method. As Reiss explained:
“The community is really grappling with understanding the meaning of this discrepancy… Both results have been tested multiple ways, so barring a series of unrelated mistakes. it is increasingly likely that this is not a bug but a feature of the universe.”
These latest results therefore suggest that some previously unknown force or some new physics might be at work in the Universe. In terms of explanations, Reiss and his team have offered three possibilities, all of which have to do with the 95% of the Universe that we cannot see (i.e. dark matter and dark energy). In 2011, Reiss and two other scientists were awarded the Nobel Prize in Physics for their 1998 discovery that the Universe was in an accelerated rate of expansion.
Consistent with that, they suggest that Dark Energy could be pushing galaxies apart with increasing strength. Another possibility is that there is an undiscovered subatomic particle out there that is similar to a neutrino, but interacts with normal matter by gravity instead of subatomic forces. These “sterile neutrinos” would travel at close to the speed of light and could collectively be known as “dark radiation”.
Any of these possibilities would mean that the contents of the early Universe were different, thus forcing a rethink of our cosmological models. At present, Riess and colleagues don’t have any answers, but plan to continue fine-tuning their measurements. So far, the SHoES team has decreased the uncertainty of the Hubble Constant to 2.3%.
This is in keeping with one of the central goals of the Hubble Space Telescope, which was to help reduce the uncertainty value in Hubble’s Constant, for which estimates once varied by a factor of 2.
So while this discrepancy opens the door to new and challenging questions, it also reduces our uncertainty substantially when it comes to measuring the Universe. Ultimately, this will improve our understanding of how the Universe evolved after it was created in a fiery cataclysm 13.8 billion years ago.
Understanding the Universe and how it has evolved over the course of billions of years is a rather daunting task. On the one hand, it involves painstakingly looking billions of light years into deep space (and thus, billions of years back in time) to see how its large-scale structure changed over time. Then, massive amounts of computing power are needed to simulate what it should look like (based on known physics) and seeing if they match up.
That is what a team of astrophysicists from the University of Zurich (UZH) did using the “Piz Daint” supercomputer. With this sophisticated machine, they simulated the formation of our entire Universe and produced a catalog of about 25 billion virtual galaxies. This catalog will be launched aboard the ESA’s Euclid mission in 2020, which will spend six years probing the Universe for the sake of investigating dark matter.
The team’s work was detailed in a study that appeared recently in the journal Computational Astrophysics and Cosmology. Led by Douglas Potter, the team spent the past three years developing an optimized code to describe (with unprecedented accuracy) the dynamics of dark matter as well as the formation of large-scale structures in the Universe.
The code, known as PKDGRAV3, was specifically designed to optimally use the available memory and processing power of modern super-computing architectures. After being executed on the “Piz Daint” supercomputer – located at the Swiss National Computing Center (CSCS) – for a period of only 80 hours, it managed to generate a virtual Universe of two trillion macro-particles, from which a catalogue of 25 billion virtual galaxies was extracted.
Intrinsic to their calculations was the way in which dark matter fluid would have evolved under its own gravity, thus leading to the formation of small concentrations known as “dark matter halos”. It is within these halos – a theoretical component that is thought to extend well beyond the visible extent of a galaxy – that galaxies like the Milky Way are believed to have formed.
Naturally, this presented quite the challenge. It required not only a precise calculation of how the structure of dark matter evolves, but also required that they consider how this would influence every other part of the Universe. As Joachim Stadel, a professor with the Center for Theoretical Astrophysics and Cosmology at UZH and a co-author on the paper, told Universe Today via email:
“We simulated 2 trillion such dark matter “pieces”, the largest calculation of this type that has ever been performed. To do this we had to use a computation technique known as the “fast multipole method” and use one of the fastest computers in the world, “Piz Daint” at the Swiss National Supercomputing Centre, which among other things has very fast graphics processing units (GPUs) which allow an enormous speed-up of the floating point calculations needed in the simulation. The dark matter clusters into dark matter “halos” which in turn harbor the galaxies. Our calculation accurately produces the distribution and properties of the dark matter, including the halos, but the galaxies, with all of their properties, must be placed within these halos using a model. This part of the task was performed by our colleagues at Barcelona under the direction of Pablo Fossalba and Francisco Castander. These galaxies then have the expected colors, spatial distribution and the emission lines (important for the spectra observed by Euclid) and can be used to test and calibrate various systematics and random errors within the entire instrument pipeline of Euclid.”
Thanks to the high precision of their calculations, the team was able to turn out a catalog that met the requirements of the European Space Agency’s Euclid mission, whose main objective is to explore the “dark universe”. This kind of research is essential to understanding the Universe on the largest of scales, mainly because the vast majority of the Universe is dark.
Between the 23% of the Universe which is made up of dark matter and the 72% that consists of dark energy, only one-twentieth of the Universe is actually made up of matter that we can see with normal instruments (aka. “luminous” or baryonic matter). Despite being proposed during the 1960s and 1990s respectively, dark matter and dark energy remain two of the greatest cosmological mysteries.
Given that their existence is required in order for our current cosmological models to work, their existence has only ever been inferred through indirect observation. This is precisely what the Euclid mission will do over the course of its six year mission, which will consist of it capturing light from billions of galaxies and measuring it for subtle distortions caused by the presence of mass in the foreground.
Much in the same way that measuring background light can be distorted by the presence of a gravitational field between it and the observer (i.e. a time-honored test for General Relativity), the presence of dark matter will exert a gravitational influence on the light. As Stadel explained, their simulated Universe will play an important role in this Euclid mission – providing a framework that will be used during and after the mission.
“In order to forecast how well the current components will be able to make a given measurement, a Universe populated with galaxies as close as possible to the real observed Universe must be created,” he said. “This ‘mock’ catalogue of galaxies is what was generated from the simulation and will be now used in this way. However, in the future when Euclid begins taking data, we will also need to use simulations like this to solve the inverse problem. We will then need to be able to take the observed Universe and determine the fundamental parameters of cosmology; a connection which currently can only be made at a sufficient precision by large simulations like the one we have just performed. This is a second important aspect of how such simulation work [and] is central to the Euclid mission.”
From the Euclid data, researchers hope to obtain new information on the nature of dark matter, but also to discover new physics that goes beyond the Standard Model of particle physics – i.e. a modified version of general relativity or a new type of particle. As Stadel explained, the best outcome for the mission would be one in which the results do not conform to expectations.
“While it will certainly make the most accurate measurements of fundamental cosmological parameters (such as the amount of dark matter and energy in the Universe) far more exciting would be to measure something that conflicts or, at the very least, is in tension with the current ‘standard lambda cold dark matter‘ (LCDM) model,” he said. “One of the biggest questions is whether the so called ‘dark energy’ of this model is actually a form of energy, or whether it is more correctly described by a modification to Einstein’s general theory of relativity. While we may just begin to scratch the surface of such questions, they are very important and have the potential to change physics at a very fundamental level.”
In the future, Stadel and his colleagues hope to be running simulations on cosmic evolution that take into account both dark matter and dark energy. Someday, these exotic aspects of nature could form the pillars of a new cosmology, one which reaches beyond the physics of the Standard Model. In the meantime, astrophysicists from around the world will likely be waiting for the first batch of results from the Euclid mission with baited breath.
Euclid is one of several missions that is currently engaged in the hunt for dark matter and the study of how it shaped our Universe. Others include the Alpha Magnetic Spectrometer (AMS-02) experiment aboard the ISS, the ESO’s Kilo Degree Survey (KiDS), and CERN’s Large Hardon Collider. With luck, these experiments will reveal pieces to the cosmological puzzle that have remained elusive for decades.
Whenever we talk about the expanding Universe, everyone wants to know how this is going to end. Sure, they say, the fact that most of the galaxies we can see are speeding away from us in all directions is really interesting. Sure, they say, the Big Bang makes sense, in that everything was closer together billions of years ago.
But how does it end? Does this go on forever? Do galaxies eventually slow down, come to a stop, and then hurtle back together in a Big Crunch? Will we get a non-stop cycle of Big Bangs, forever and ever?
We’ve done a bunch of articles on many different aspects of this question, and the current conclusion astronomers have reached is that because the Universe is flat, it’s never going to collapse in on itself and start another Big Bang.
But wait, what does it mean to say that the Universe is “flat”? Why is that important, and how do we even know?
Before we can get started talking about the flatness of the Universe, we need to talk about flatness in general. What does it mean to say that something is flat?
If you’re in a square room and walk around the corners, you’ll return to your starting point having made 4 90-degree turns. You can say that your room is flat. This is Euclidian geometry.
But if you make the same journey on the surface of the Earth. Start at the equator, make a 90-degree turn, walk up to the North Pole, make another 90-degree turn, return to the equator, another 90-degree turn and return to your starting point.
In one situation, you made 4 turns to return to your starting point, in another situation it only took 3. That’s because the topology of the surface you were walking on decided what happens when you take a 90-degree turn.
You can imagine an even more extreme example, where you’re walking around inside a crater, and it takes more than 4 turns to return to your starting point.
Another analogy, of course, is the idea of parallel lines. If you fire off two parallel lines at the North pole, they move away from each other, following the topology of the Earth and then come back together.
Got that? Great.
Now, what about the Universe itself? You can imagine that same analogy. Imaging flying out into space on a rocket for billions of light-years, performing 90-degree maneuvers and returning to your starting point.
You can’t do it in 3, or 5, you need 4, which means that the topology of the Universe is flat. Which is totally intuitive, right? I mean, that would be your assumption.
But astronomers were skeptical and needed to know for certain, and so, they set out to test this assumption.
In order to prove the flatness of the Universe, you would need to travel a long way. And astronomers use the largest possible observation they can make. The Cosmic Microwave Background Radiation, the afterglow of the Big Bang, visible in all directions as a red-shifted, fading moment when the Universe became transparent about 380,000 years after the Big Bang.
When this radiation was released, the entire Universe was approximately 2,700 C. This was the moment when it was cool enough for photons were finally free to roam across the Universe. The expansion of the Universe stretched these photons out over their 13.8 billion year journey, shifting them down into the microwave spectrum, just 2.7 degrees above absolute zero.
With the most sensitive space-based telescopes they have available, astronomers are able to detect tiny variations in the temperature of this background radiation.
And here’s the part that blows my mind every time I think about it. These tiny temperature variations correspond to the largest scale structures of the observable Universe. A region that was a fraction of a degree warmer become a vast galaxy cluster, hundreds of millions of light-years across.
The Cosmic Microwave Background Radiation just gives and gives, and when it comes to figuring out the topology of the Universe, it has the answer we need. If the Universe was curved in any way, these temperature variations would appear distorted compared to the actual size that we see these structures today.
But they’re not. To best of its ability, ESA’s Planck space telescope, can’t detect any distortion at all. The Universe is flat.
Well, that’s not exactly true. According to the best measurements astronomers have ever been able to make, the curvature of the Universe falls within a range of error bars that indicates it’s flat. Future observations by some super Planck telescope could show a slight curvature, but for now, the best measurements out there say… flat.
We say that the Universe is flat, and this means that parallel lines will always remain parallel. 90-degree turns behave as true 90-degree turns, and everything makes sense.
But what are the implications for the entire Universe? What does this tell us?
Unfortunately, the biggest thing is what it doesn’t tell us. We still don’t know if the Universe is finite or infinite. If we could measure its curvature, we could know that we’re in a finite Universe, and get a sense of what its actual true size is, out beyond the observable Universe we can measure.
We know that the volume of the Universe is at least 100 times more than we can observe. At least. If the flatness error bars get brought down, the minimum size of the Universe goes up.
And remember, an infinite Universe is still on the table.
Another thing this does, is that it actually causes a problem for the original Big Bang theory, requiring the development of a theory like inflation.
Since the Universe is flat now, it must have been flat in the past, when the Universe was an incredibly dense singularity. And for it to maintain this level of flatness over 13.8 billion years of expansion, in kind of amazing.
In fact, astronomers estimate that the Universe must have been flat to 1 part within 1×10^57 parts.
Which seems like an insane coincidence. The development of inflation, however, solves this, by expanding the Universe an incomprehensible amount moments after the Big Bang. Pre and post inflation Universes can have vastly different levels of curvature.
In the olden days, cosmologists used to say that the flatness of the Universe had implications for its future. If the Universe was curved where you could complete a full journey with less than 4 turns, that meant it was closed and destined to collapse in on itself.
And it was more than 4 turns, it was open and destined to expand forever.
Well, that doesn’t really matter any more. In 1998, the astronomers discovered dark energy, which is this mysterious force accelerating the expansion of the Universe. Whether the Universe is open, closed or flat, it’s going to keep on expanding. In fact, that expansion is going to accelerate, forever.
I hope this gives you a little more understanding of what cosmologists mean when they say that the Universe is flat. And how do we know it’s flat? Very precise measurements in the Cosmic Microwave Background Radiation.
Is there anything that all pervasive relic of the early Universe can’t do?