Illustration of the stellar halo surrounding our Milky Way Galaxy. Credit: Melissa Weiss / Harvard & Smithsonian’s Center for Astrophysics
Sometimes in astronomy, a simple question has a difficult answer. One such question is this: what is the mass of our galaxy?
On Earth, we usually determine the mass of an object by placing it on a scale or balance. The weight of an object in Earth’s gravitational field lets us determine the mass. But we can’t put the Milky Way on a scale. Another difficulty with massing our galaxy is that there are two types of mass. There is the mass of dark matter that makes up most of the Milky Way’s mass, and there is all the regular matter like stars, planets, and us, which is known as baryonic matter.
Artist's impression of Dark matter in the Coma Cluster region. Credit: HyeongHan et al.
According to our predominant cosmological models, Dark Matter makes up the majority of mass in the Universe (roughly 85%). While it is not detectable in visible light, its influence can be seen based on how it causes matter to form large-scale structures in our Universe. Based on ongoing observations, astronomers have determined that Dark Matter structures are filamentary, consisting of long, thin strands. For the first time, using the Subaru Telescope, a team of astronomers directly detected Dark Matter filaments in a massive galaxy cluster, providing new evidence to test theories about the evolution of the Universe.
Artist's impression of the huge halo of hot gas surrounding the Milky Way Galaxy. Credit: NASA
In the past decade and a half, hundreds of Fast Radio Bursts (FRBs) have been detected by astronomers. These transient energetic bursts occur suddenly, typically last for just a few milliseconds, and are rarely seen again (except in the rare case of repeating bursts). While astronomers are still not entirely sure what causes this phenomenon, FRBs have become a tool for astronomers hoping to map out the cosmos. Based on the way radio emissions are dispersed as they travel through space, astronomers can measure the structure and distribution of matter in and around galaxies.
Using the Deep Synoptic Array (DSA) at the Owens Valley Radio Observatory (OVRO), a team of astronomers from Caltech and Cornell University used an intense FRB from a nearby galaxy to probe the halo of hot gas that surrounds the Milky Way. Their results show that our galaxy has significantly less visible (“baryonic” or “normal”) matter than previously expected. These findings support theories that matter is regularly ejected from our galaxy due to stellar winds, supernovae, and accreting supermassive black holes (SMBHs).
This image shows the sky around the ultra diffuse galaxy NGC 1052-DF2. It was created from images forming part of the Digitized Sky Survey 2. NGC 1052-DF2 is basically invisible in this image. It is located to the southwest of the bright elliptical galaxy NGC 1052, which is dominating the field of view, and east of the bright red star HD 16873. Credit:
ESA/Hubble, NASA, Digitized Sky Survey 2
Acknowledgement: Davide de Martin
One of the greatest cosmological mysteries facing astrophysicists today is Dark Matter. Since the 1960s, scientists have postulated that this invisible mass accounts for most of the matter in the Universe. While there are still many unresolved questions about it – i.e., What is it composed of? How do we detect it? What evidence is there beyond indirect detection? – we have managed to learn a few things about it over time.
For example, astrophysicists have observed that Dark Matter played a vital role in the formation of galaxies and is responsible for keeping them gravitationally bound. However, when an international team of astronomers observed the ultra-diffuse galaxy AGC 114905, they found no evidence of Dark Matter at all. If these observations are accurate, this discovery could force scientists to reevaluate their cosmological models and the way we look at the Universe.
An illustration of cosmic expansion. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab
About 25 years ago, astrophysicists noticed something very interesting about the Universe. The fact that it was in a state of expansion had been known since the 1920s, thanks to the observation of Edwin Hubble. But thanks to the observations astronomers were making with the space observatory that bore his name (the Hubble Space Telescope), they began to notice how the rate of cosmic expansion was getting faster!
This has led to the theory that the Universe is filled with an invisible and mysterious force, known as Dark Energy (DE). Decades after it was proposed, scientists are still trying to pin down this elusive force that makes up about 70% of the energy budget of the Universe. According to a recent study by an international team of researchers, the XENON1T experiment may have already detected this elusive force, opening new possibilities for future DE research.
Where is all the missing matter? That question has plagued astronomers for decades, because the Universe looks emptier than it should, given current theories about its makeup. Most of the Universe (70%) appears to be composed of Dark Energy, the mysterious force which is causing the Universe’s rate of expansion to increase. Another 25% of the Universe is Dark Matter, an unknown substance which cannot be seen, but has been theorized to explain the otherwise inexplicable gravitational forces which govern the formation of galaxies. That leaves Baryonic Matter – all the normal ‘stuff’ like you, me, the trees, the planets, and the stars – to make up just 5% of the Universe. But when astronomers look out into the sky, there doesn’t even seem to be enough normal matter to make up 5%. Some of the normal matter is missing!
A simulation of the cosmic web, diffuse tendrils of gas that connect galaxies across the universe. Credit: Illustris Collaboration
In the 1960s, astronomers began to notice that the Universe appeared to be missing some mass. Between ongoing observations of the cosmos and the the Theory of General Relativity, they determined that a great deal of the mass in the Universe had to be invisible. But even after the inclusion of this “dark matter”, astronomers could still only account for about two-thirds of all the visible (aka. baryonic) matter.
This gave rise to what astrophysicists dubbed the “missing baryon problem”. But at long last, scientists have found what may very well be the last missing normal matter in the Universe. According to a recent study by a team of international scientists, this missing matter consists of filaments of highly-ionized oxygen gas that lies in the space between galaxies.
Artist’s impression of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Credit: ESO/M. Kornmesser
For the sake of their study, the team consulted data from a series of instruments to examine the space near a quasar called 1ES 1553. Quasars are extremely massive galaxies with Active Galactic Nuclei (AGN) that emit tremendous amounts of energy. This energy is the result of gas and dust being accreted onto supermassive black holes (SMBHs) at the center of their galaxies, which results in the black holes emitting radiation and jets of superheated particles.
In the past, researchers believed that of the normal matter in the Universe, roughly 10% was bound up in galaxies while 60% existed in diffuse clouds of gas that fill the vast spaces between galaxies. However, this still left 30% of normal matter unaccounted for. This study, which was the culmination of a 20-year search, sought to determine if the last baryons could also be found in intergalactic space.
This theory was suggested by Charles Danforth, a research associate at CU Boulder and a co-author on this study, in a 2012 paper that appeared in The Astrophysical Journal – titled “The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still be Missing“. In it, Danforth suggested that the missing baryons were likely to be found in the warm-hot intergalactic medium (WHIM), a web-like pattern in space that exists between galaxies.
As Michael Shull – a professor of Astrophysical and Planetary Sciences at the University of Colorado Boulder and one of the co-authors on the study – indicated, this wild terrain seemed like the perfect place to look.“This is where nature has become very perverse,” he said. “This intergalactic medium contains filaments of gas at temperatures from a few thousand degrees to a few million degrees.”
Close-up of star near a supermassive black hole (artist’s impression). Credit: ESA/Hubble, ESO, M. Kornmesser
To test this theory, the team used data from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to examine the WHIM near the quasar 1ES 1553. They then used the European Space Agency’s (ESA) X-ray Multi-Mirror Mission (XMM-Newton) to look closer for signs of the baryons, which appeared in the form of highly-ionized jets of oxygen gas heated to temperatures of about 1 million °C (1.8 million °F).
First, the researchers used the COS on the Hubble Space Telescope to get an idea of where they might find the missing baryons in the WHIM. Next, they homed in on those baryons using the XMM-Newton satellite. At the densities they recorded, the team concluded that when extrapolated to the entire Universe, this super-ionized oxygen gas could account for the last 30% of ordinary matter.
As Prof. Shull indicated, these results not only solve the mystery of the missing baryons but could also shed light on how the Universe began. “This is one of the key pillars of testing the Big Bang theory: figuring out the baryon census of hydrogen and helium and everything else in the periodic table,” he said.
Looking ahead, Shull indicated that the researchers hope to confirm their findings by studying more bright quasars. Shull and Danforth will also explore how the oxygen gas got to these regions of intergalactic space, though they suspect that it was blown there over the course of billions of years from galaxies and quasars. In the meantime, however, how the “missing matter” became part of the WHIM remains an open question. As Danforth asked:
“How does it get from the stars and the galaxies all the way out here into intergalactic space?. There’s some sort of ecology going on between the two regions, and the details of that are poorly understood.”
Assuming these results are correct, scientists can now move forward with models of cosmology where all the necessary “normal matter” is accounted for, which will put us a step closer to understanding how the Universe formed and evolved. Now if we could just find that elusive dark matter and dark energy, we’d have a complete picture of the Universe! Ah well, one mystery at a time…
The structure of the universe at the largest scale. Credit: NASA, ESA, and E. Hallman (University of Colorado, Boulder)
For decades, the predominant cosmological model used by scientists has been based on the theory that in addition to baryonic matter – aka. “normal” or “luminous” matter, which we can see – the Universe also contains a substantial amount of invisible mass. This “Dark Matter” accounts for roughly 26.8% of the mass of the Universe, whereas normal matter accounts for just 4.9%.
While the search for Dark Matter is ongoing and direct evidence is yet to be found, scientists have also been aware that roughly 90% of the Universe’s normal matter still remained undetected. According to twonew studies that were recently published, much of this normal matter – which consists of filaments of hot, diffuse gas that links galaxies together – may have finally been found.
Based on cosmological simulations, the predominant theory has been that the previously-undetected normal matter of the Universe consists of strands of baryonic matter – i.e. protons, neutrons and electrons – that is floating between galaxies. These regions are what is known as the “Cosmic Web”, where low density gas exists at a temperatures of 105 to 107 K (-168 t0 -166 °C; -270 to 266 °F).
For the sake of their studies, both teams consulted data from the Planck Collaboration, a venture maintained by the European Space Agency that includes all those who contributed to the Planck mission (ESA). This was presented in 2015, where it was used to create a thermal map of the Universe by measuring the influence of the Sunyaev-Zeldovich (SZ) effect.
This effect refers to a spectral distortion in the Cosmic Microwave Background, where photons are scattered by ionized gas in galaxies and larger structures. During its mission to study the cosmos, the Planck satellite measured the spectral distortion of CMB photons with great sensitivity, and the resulting thermal map has since been used to chart the large-scale structure of the Universe.
IR map of the whole Galaxy showing the plane and bulge of the Galaxy full of stars and dust. Credit: SDSS
However, the filaments between galaxies appeared too faint for scientists to examine at the time. To remedy this, the two teams consulted data from the North and South CMASS galaxy catalogues, which were produced from the 12th data release of the Sloan Digital Sky Survey (SDSS). From this data set, they then selected pairs of galaxies and focused on the space between them.
They then stacked the thermal data obtained by Planck for these areas on top of each other in order to strengthen the signals caused by SZ effect between galaxies. As Dr. Hideki told Universe Today via email:
“The SDSS galaxy survey gives a shape of the large-scale structure of the Universe. The Planck observation provides an all-sky map of gas pressure with a better sensitivity. We combine these data to probe the low-dense gas in the cosmic web.”
While Tanimura and his team stacked data from 260,000 galaxy pairs, de Graaff and her team stacked data from over a million. In the end, the two teams came up with strong evidence of gas filaments, though their measurements differed somewhat. Whereas Tanimura’s team found that the density of these filaments was around three times the average density in the surrounding void, de Graaf and her team found that they were six times the average density.
“We detect the low-dense gas in the cosmic web statistically by a stacking method,” said Hideki. “The other team uses almost the same method. Our results are very similar. The main difference is that we are probing a nearby Universe, on the other hand, they are probing a relatively farther Universe.”
This illustration shows the evolution of the Universe, from the Big Bang on the left, to modern times on the right. Image: NASA
This particular aspect of particularly interesting, in that it hints that over time, baryonic matter in the Cosmic Web has become less dense. Between these two results, the studies accounted for between 15 and 30% of the total baryonic content of the Universe. While that would mean that a significant amount of the Universe’s baryonic matter still remains to be found, it is nevertheless an impressive find.
As Hideki explained, their results not only support the current cosmological model of the Universe (the Lambda CDM model) but also goes beyond it:
“The detail in our universe is still a mystery. Our results shed light on it and reveals a more precise picture of the Universe. When people went out to the ocean and started making a map of our world, it was not used for most of the people then, but we use the world map now to travel abroad. In the same way, a map of the entire universe may not be valuable now because we do not have a technology to go far out to the space. However, it could be valuable 500 years later. We are in the first stage of making a map of the entire Universe.”
It also opens up opportunities for future studies of the Comsic Web, which will no doubt benefit from the deployment of next-generation instruments like James Webb Telescope, the Atacama Cosmology Telescope and the Q/U Imaging ExperimenT (QUIET). With any luck, they will be able to spot the remaining missing matter. Then, perhaps we can finally zero in on all the invisible mass!
A section of the 3D map constructed by BOSS. The rectangle on the far left shows a cutout of 1000 sq. degrees in the sky containing nearly 120,000 galaxies, or roughly 10% of the total survey. Credit: Jeremy Tinker/SDSS-III
In 1929, Edwin Hubble forever changed our understanding of the cosmos by showing that the Universe is in a state of expansion. By the 1990s, astronomers determined that the rate at which it is expanding is actually speeding up, which in turn led to the theory of “Dark Energy“. Since that time, astronomers and physicists have sought to determine the existence of this force by measuring the influence it has on the cosmos.
The latest in these efforts comes from the Sloan Digital Sky Survey III (SDSS III), where an international team of researchers have announced that they have finished creating the most precise measurements of the Universe to date. Known as the Baryon Oscillation Spectroscopic Survey (BOSS), their measurements have placed new constraints on the properties of Dark Energy.
The new measurements were presented by Harvard University astronomer Daniel Eisenstein at a recent meeting of the American Astronomical Society. As the director of the Sloan Digital Sky Survey III (SDSS-III), he and his team have spent the past ten years measuring the cosmos and the periodic fluctuations in the density of normal matter to see how galaxies are distributed throughout the Universe.
An illustration of baryon acoustic oscillations, which are imprinted in the early universe and can still be seen today in galaxy surveys like BOSS. Credit: Chris Blake and Sam Moorfield
And after a decade of research, the BOSS team was able to produce a three-dimensional map of the cosmos that covers more than six billion light-years. And while other recent surveys have looked further afield – up to distances of 9 and 13 billion light years – the BOSS map is unique in that it boasts the highest accuracy of any cosmological map.
In fact, the BOSS team was able to measure the distribution of galaxies in the cosmos, and at a distance of 6 billion light-years, to within an unprecedented 1% margin of error. Determining the nature of cosmic objects at great distances is no easy matter, due the effects of relativity. As Dr. Eisenstein told Universe Today via email:
“Distances are a long-standing challenge in astronomy. Whereas humans often can judge distance because of our binocular vision, galaxies beyond the Milky Way are much too far away to use that. And because galaxies come in a wide range of intrinsic sizes, it is hard to judge their distance. It’s like looking at a far-away mountain; one’s judgement of its distance is tied up with one’s judgement of its height.”
In the past, astronomers have made accurate measurements of objects within the local universe (i.e. planets, neighboring stars, star clusters) by relying on everything from radar to redshift – the degree to which the wavelength of light is shifted towards the red end of the spectrum. However, the greater the distance of an object, the greater the degree of uncertainty.
An artist’s concept of the latest, highly accurate measurement of the Universe from BOSS. Credit: Zosia Rostomian/Lawrence Berkeley National Laboratory
And until now, only objects that are a few thousand light-years from Earth – i.e. within the Milky Way galaxy – have had their distances measured to within a one-percent margin of error. As the largest of the four projects that make up the Sloan Digital Sky Survey III (SDSS-III), what sets BOSS apart is the fact that it relies primarily on the measurement of what are called “baryon acoustic oscillations” (BAOs).
These are essentially subtle periodic ripples in the distribution of visible baryonic (i.e. normal) matter in the cosmos. As Dr. Daniel Eisenstein explained:
“BOSS measures the expansion of the Universe in two primary ways. The first is by using the baryon acoustic oscillations (hence the name of the survey). Sound waves traveling in the first 400,000 years after the Big Bang create a preferred scale for separations of pairs of galaxies. By measuring this preferred separation in a sample of many galaxies, we can infer the distance to the sample.
“The second method is to measure how clustering of galaxies differs between pairs oriented along the line of sight compared to transverse to the line of sight. The expansion of the Universe can cause this clustering to be asymmetric if one uses the wrong expansion history when converting redshifts to distance.”
With these new, highly-accurate distance measurements, BOSS astronomers will be able to study the influence of Dark Matter with far greater precision. “Different dark energy models vary in how the acceleration of the expansion of the Universe proceeds over time,” said Eisenstein. “BOSS is measuring the expansion history, which allows us to infer the acceleration rate. We find results that are highly consistent with the predictions of the cosmological constant model, that is, the model in which dark energy has a constant density over time.”
Discerning the large-scale structure of the universe, and the role played by Dark Energy, is key to unlocking its mysteries. Credit: NAOJ/CFHT/ SDSS
In addition to measuring the distribution of normal matter to determine the influence of Dark Energy, the SDSS-III Collaboration is working to map the Milky Way and search for extrasolar planets. The BOSS measurements are detailed in a series of articles that were submitted to journals by the BOSS collaboration last month, all of which are now available online.
And BOSS is not the only effort to understand the large-scale structure of our Universe, and how all its mysterious forces have shaped it. Just last month, Professor Stephen Hawking announced that the COSMOS supercomputing center at Cambridge University would be creating the most detailed 3D map of the Universe to date.
Relying on data obtained by the CMB data obtained by the ESA’s Planck satellite and information from the Dark Energy Survey, they also hope to measure the influence Dark Energy has had on the distribution of matter in our Universe. Who knows? In a few years time, we may very well come to understand how all the fundamental forces governing the Universe work together.
