astrophysics Archives

October 8, 2015February 23, 2017 by Fraser Cain

What are White Holes?

Black holes are created when stars die catastrophically in a supernova. So what in the universe is a white hole?

It’s imagination day, and we’re going to talk about fantasy creatures. Like unicorns, but even rarer. Like leprechauns, but even more fantastical!

Today, we’re going to talk about white holes. Before we talk about white holes, let’s talk about black holes. And before we talk about Black Holes, what’s is this thing you have with holes exactly?

Black holes are places in the Universe where matter and energy are compacted so densely together that their escape velocity is greater than the speed of light. We’ve done at least a million videos on them, but if you still want more info, you can start here with our Black Hole playlist.

Fully describing a black hole requires a lot of fancy math, but these are real objects in our Universe. They were predicted by Einstein’s theory of relativity, and actually discovered over the last few decades.

Black holes are created when stars, much more massive than our Sun, die catastrophically in a supernova.
So then what’s a white hole?

White holes are created when astrophysicists mathematically explore the environment around black holes, but pretend there’s no mass within the event horizon. What happens when you have a black hole singularity with no mass?

White holes are completely theoretical mathematical concepts. In fact, if you do black hole mathematics for a living, I’m told, ignoring the mass of the singularity makes your life so much easier.

They’re not things that actually exist. It’s not like astronomers detected an unusual outburst of radiation and then developed hypothetical white hole models to explain them.

As my good friend and sometimes Guide to Space contributor, Dr. Brian Koberlein says, “If you start with five cupcakes and start giving them away, you eventually run out. At that point you can’t give away any more. In this case you can’t count down past zero. Sure, you can hand out slips of paper with “I O U ONE cupcake.” written on them, but it would be ridiculous to use the existence of negative numbers to claim that “negative cupcakes” exist and can be handed out to people.”

Now if white holes did exist, which they probably don’t, they would behave like reverse black holes – just like the math predicts. Instead of pulling material inward, a white hole would blast material out into space like some kind of white chocolate fountain. So generous, these white holes and their chocolate.

One of the other implications of white hole math, is that they only theoretically exist as long as there isn’t a single speck of matter within the event horizon. As soon as single atom of hydrogen drifted into the region, the whole thing would collapse. Even if white holes were created back at the beginning of the Universe, they would have collapsed long ago, since our Universe is already filled with stray matter.

That said, there are a few physicists out there who think white holes might be more than theoretical. Hal Haggard and Carlo Rovelli of Aix-Marseille University in France are working to explain what happens within black holes using a branch of theoretical physics called loop quantum gravity.

In theory, a black hole singularity would compress down until the smallest possible size predicted by physics. Then it would rebound as a white hole. But because of the severe time dilation effect around a black hole, this event would take billions of years for even the lowest mass ones to finally get around to popping.

If there were microscopic black holes created after the Big Bang, they might get around to decaying and exploding as white holes any day now. Except, according to Stephen Hawking, they would have already evaporated.

Another interesting idea put forth by physicists, is that a white hole might explain the Big Bang, since this is another situation where a tremendous amount of matter and energy spontaneously appeared.

In all likelihood, white holes are just fancy math. And since fancy math rarely survives contact with reality, white holes are probably just imaginary.

What other highly theoretical theories in space and physics would you like us to investigate? Tell us in the comments below.

December 15, 2014December 23, 2015 by Vanessa Janek

New Signal May Be Evidence of Dark Matter, Say Researchers

Dark Matter Halo and dwarf galaxies — All galaxies are thought to have a dark matter halo. This image shows the distribution of dark matter surrounding our very own Milky Way. Image credit: J. Diemand, M. Kuhlen and P. Madau (UCSC)

Dark matter is the architect of large-scale cosmic structure and the engine behind proper rotation of galaxies. It’s an indispensable part of the physics of our Universe – and yet scientists still don’t know what it’s made of. The latest data from Planck suggest that the mysterious substance comprises 26.2% of the cosmos, making it nearly five and a half times more prevalent than normal, everyday matter. Now, four European researchers have hinted that they may have a discovery on their hands: a signal in x-ray light that has no known cause, and may be evidence of a long sought-after interaction between particles – namely, the annihilation of dark matter.

When astronomers want to study an object in the night sky, such as a star or galaxy, they begin by analyzing its light across all wavelengths. This allows them to visualize narrow dark lines in the object’s spectrum, called absorption lines. Absorption lines occur because a star’s or galaxy’s component elements soak up light at certain wavelengths, preventing most photons with those energies from reaching Earth. Similarly, interacting particles can also leave emission lines in a star’s or galaxy’s spectrum, bright lines that are created when excess photons are emitted via subatomic processes such as excitement and decay. By looking closely at these emission lines, scientists can usually paint a robust picture of the physics going on elsewhere in the cosmos.

But sometimes, scientists find an emission line that is more puzzling. Earlier this year, researchers at the Laboratory of Particle Physics and Cosmology (LPPC) in Switzerland and Leiden University in the Netherlands identified an excess bump of energy in x-ray light coming from both the Andromeda galaxy and the Perseus star cluster: an emission line with an energy around 3.5keV. No known process can account for this line; however, it is consistent with models of the theoretical sterile neutrino – a particle that many scientists believe is a prime candidate for dark matter.

The researchers believe that this strange emission line could result from the annihilation, or decay, of these dark matter particles, a process that is thought to release x-ray photons. In fact, the signal appeared to be strongest in the most dense regions of Andromeda and Perseus and increasingly more diffuse away from the center, a distribution that is also characteristic of dark matter. Additionally, the signal was absent from the team’s observations of deep, empty space, implying that it is real and not just instrumental artifact.

In a pre-print of their paper, the researchers are careful to stress that the signal itself is weak by scientific standards. That is, they can only be 99.994% sure that it is a true result and not just a rogue statistical fluctuation, a level of confidence that is known as 4σ. (The gold standard for a discovery in science is 5σ: a result that can be declared “true” with 99.9999% confidence) Other scientists are not so sure that dark matter is such a good explanation after all. According to predictions made based on measurements of the Lyman-alpha forest – that is, the spectral pattern of hydrogen absorption and photon emission within very distant, very old gas clouds – any particle purporting to be dark matter should have an energy above 10keV – more than twice the energy of this most recent signal.

As always, the study of cosmology is fraught with mysteries. Whether this particular emission line turns out to be evidence of a sterile neutrino (and thus of dark matter) or not, it does appear to be a signal of some physical process that scientists do not yet understand. If future observations can increase the certainty of this discovery to the 5σ level, astrophysicists will have yet another phenomena to account for – an exciting prospect, regardless of the final result.

The team’s research has been accepted to Physical Review Letters and will be published in an upcoming issue.

November 4, 2014December 23, 2015 by Nancy Atkinson

Mysterious Object “G2” at Galactic Center is Actually Binary Star

An image from W. M. Keck Observatory near infrared data shows that G2 survived its closest approach to the black hole and continues happily on its orbit. The green circle just to its right depicts the location of the invisible supermassive black hole. Credit: Andrea Ghez, Gunther Witzel/UCLA Galactic Center Group/W. M. Keck Observatory

A mysterious object swinging around the supermassive black hole in the center our galaxy has surprised astronomers by actually surviving what many thought would be a devastating encounter. And with its survival, researchers have finally been able to solve the conundrum of what the object – known as G2 — actually is. Since G2 was discovered in 2011, there was a debate whether it was a huge cloud of hydrogen gas or a star surrounded by gas. Turns out, it was neither … or actually, all of the above, and more.

Astronomers now say that G2 is most likely a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust.

“G2 survived and continued happily on its orbit; a simple gas cloud would not have done that,” said Andrea Ghez from UCLA, who has led the observations of G2. “G2 was basically unaffected by the black hole. There were no fireworks.”

This was one of the “most watched” recent events in astronomy, since it was the first time astronomers have been able to view an encounter with a black hole like this in “real time.” The thought was that watching G2’s demise would not only reveal what this object was, but also provide more information on how matter behaves near black holes and how supermassive black holes “eat” and evolve.

The two Keck 10-meter domes atop Mauna Kea. (Rick Peterson/WMKO)

Using the Keck Observatory, Ghez and her team have been able to keep an eye on G2’s movements and how the black hole’s powerful gravitational field affected it.

While some researchers initially thought G2 was a gas cloud, others argued that they weren’t seeing the amount of stretching or “spaghettification” that would be expected if this was just a cloud of gas.

As Ghez told Universe Today earlier this year, she thought it was a star. “Its orbit looks so much like the orbits of other stars,” she said. “There’s clearly some phenomenon that is happening, and there is some layer of gas that’s interacting because you see the tidal stretching, but that doesn’t prevent a star being in the center.”

Now, after watching the object the past few months, Ghez said G2 appears to be just one of an emerging class of stars near the black hole that are created because the black hole’s powerful gravity drives binary stars to merge into one. She also noted that, in our galaxy, massive stars primarily come in pairs. She says the star suffered an abrasion to its outer layer but otherwise will be fine.

Ghez explained in a UCLA press release that when two stars near the black hole merge into one, the star expands for more than 1 million years before it settles back down.

“This may be happening more than we thought. The stars at the center of the galaxy are massive and mostly binaries,” she said. “It’s possible that many of the stars we’ve been watching and not understanding may be the end product of mergers that are calm now.”

Ghez and her colleagues also determined that G2 appears to be in that inflated stage now and is still undergoing some spaghettification, where it is being elongated. At the same time, the gas at G2’s surface is being heated by stars around it, creating an enormous cloud of gas and dust that has shrouded most of the massive star.

Usually in astrophysics, timescales of events taking place are very long — not over the course of several months. But it’s important to note that G2 actually made this journey around the galactic center around 25,000 years ago. Because of the amount of time it takes light to travel, we can only now observe this event which happened long ago.

“We are seeing phenomena about black holes that you can’t watch anywhere else in the universe,” Ghez added. “We are starting to understand the physics of black holes in a way that has never been possible before.”

The research has been published in the journal Astrophysical Journal Letters.

The Physics Behind “Interstellar’s” Visual Effects Was So Good, it Led to a Scientific Discovery

Kip Thorne’s concept for a black hole in 'Interstellar.' Image Credit: Paramount Pictures

While he was working on the film Interstellar, executive producer Kip Thorne was tasked with creating the black hole that would be central to the plot. As a theoretical physicist, he also wanted to create something that was truly realistic and as close to the real thing as movie-goers would ever see.

On the other hand, Christopher Nolan – the film’s director – wanted to create something that would be a visually-mesmerizing experience. As you can see from the image above, they certainly succeeded as far as the aesthetics were concerned. But even more impressive was how the creation of this fictitious black hole led to an actual scientific discovery.

Continue reading “The Physics Behind “Interstellar’s” Visual Effects Was So Good, it Led to a Scientific Discovery”

September 24, 2014December 23, 2015 by Jason Major

There Are No Such Things As Black Holes

UNC-Chapel Hill physics professor Laura Mersini-Houghton has proven mathematically that black holes don't exist. (Source: unc.edu)

That’s the conclusion reached by one researcher from the University of North Carolina: black holes can’t exist in our Universe — not mathematically, anyway.

“I’m still not over the shock,” said Laura Mersini-Houghton, associate physics professor at UNC-Chapel Hill. “We’ve been studying this problem for a more than 50 years and this solution gives us a lot to think about.”

In a news article spotlighted by UNC the scenario suggested by Mersini-Houghton is briefly explained. Basically, when a massive star reaches the end of its life and collapses under its own gravity after blasting its outer layers into space — which is commonly thought to result in an ultra-dense point called a singularity surrounded by a light- and energy-trapping event horizon — it undergoes a period of intense outgoing radiation (the sort of which was famously deduced by Stephen Hawking.) This release of radiation is enough, Mersini-Houghton has calculated, to cause the collapsing star to lose too much mass to allow a singularity to form. No singularity means no event horizon… and no black hole.

Artist's conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library — Artist’s conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library

At least, not by her numbers.

Read more: How Do Black Holes Form?

So what does happen to massive stars when they die? Rather than falling ever inwards to create an infinitely dense point hidden behind a space-time “firewall” — something that, while fascinating to ponder and a staple of science fiction, has admittedly been notoriously tricky for scientists to reconcile with known physics — Mersini-Houghton suggests that they just “probably blow up.” (Source)

According to the UNC article Mersini-Houghton’s research “not only forces scientists to reimagine the fabric of space-time, but also rethink the origins of the universe.”

Hm.

The submitted papers on this research are publicly available on arXiv.org and can be found here and here.

Don’t believe it? I’m not surprised. I’m certainly no physicist but I do expect that there will be many scientists (and layfolk) who’ll have their own take on Mersini-Houghton’s findings (*ahem* Brian Koberlein*) especially considering 1. the popularity of black holes in astronomical culture, and 2. the many — scratch that; the countless — observations that have been made on quite black hole-ish objects found throughout the Universe.

So what do you think? Have black holes just been voted off the cosmic island? Or are the holes more likely in the research? Share your thoughts in the comments!

Want to hear more from Mersini-Houghton herself? Here’s a link to a video explaining her view of why event horizons and singularities might simply be a myth.

Source: UNC-Chapel Hill. HT to Marco Iozzi on the Google+ Space Community (join us!)

Of course this leads me to ask: if there really are “no black holes” then what’s causing the stars in the center of our galaxy to move like this?

*Added Sept. 25: I knew Brian wouldn’t disappoint! Read his post on why “Yes, Virginia, There Are Black Holes.”

February 10, 2014December 23, 2015 by Brian Koberlein

Planck “Star” to Arise From Black Holes?

A new paper has been posted on the arxiv (a repository of research preprints) introducing the idea of a Planck star arising from a black hole. These hypothetical objects wouldn’t be a star in the traditional sense, but rather the light emitted when a black hole dies at the hands of Hawking radiation. The paper hasn’t been peer reviewed, but it presents an interesting idea and a possible observational test.

When a large star reaches the end of its life, it explodes as a supernova, which can cause its core to collapse into a black hole. In the traditional model of a black hole, the material collapses down into an infinitesimal volume known as a singularity. Of course this doesn’t take into account quantum theory.

Although we don’t have a complete theory of quantum gravity, we do know a few things. One is that black holes shouldn’t last forever. Because of quantum fluctuations near the event horizon of a black hole, a black hole will emit Hawking radiation. As a result, a black hole will gradually lose mass as it radiates. The amount of Hawking radiation it emits is inversely proportional to its size, so as the black hole gets smaller it will emit more and more Hawking radiation until it finally radiates completely away.

Because black holes don’t last forever, this has led Stephen Hawking and others to propose that black holes don’t have an event horizon, but rather an apparent horizon. This would mean the material within a black hole would not collapse into a singularity, which is where this new paper comes in.

Diagram showing how matter approaches Planck density. Credit: Carlo Rovelli and Francesca Vidotto

The authors propose that rather than collapsing into a singularity, the matter within a black hole will collapse until it is about a trillionth of a meter in size. At that point its density would be on the order of the Planck density. When the the black hole ends its life, this “Planck star” would be revealed. Because this “star” would be at the Planck density, it would radiate at a specific wavelength of gamma rays. So if they exist, a gamma ray telescope should be able to observe them.

Just to be clear, this is still pretty speculative. So far there isn’t any observational evidence that such a Planck star exists. It is, however, an interesting solution to the paradoxical side of black holes.

January 17, 2014December 23, 2015 by Shannon Hall

Enduring Quests and Daring Visions: NASA Lays Out a Roadmap for Astrophysics

An artist's concept of Kepler-69c, a super-Earth in the habitable zone of a sun-like star.

Three decades ago we were unaware that exoplanets circled other stars. We had just started talking about dark matter but remained blissfully ignorant of dark energy. The Hubble Space Telescope was still on the drawing board and our understanding of the life cycle of stars, the evolution of galaxies, and the history of the Universe was shaky.

But over the past three decades we have discovered thousands of exoplanets around other stars. We have mapped the life cycle of stars from their formation in beautiful stellar nurseries to their sometimes explosive deaths. We have seen deep into the history of the Universe allowing us to paint a picture of galaxies growing from mere shreds to the incredible spiral structures we see today. We now believe dark matter dominates the underlying framework of the Universe, while dark energy drives its accelerating expansion.

The amount of growth over the past three decades has been dramatic. To better access what the next three decades will bring, NASA has laid out a roadmap — a long-term vision for future missions — necessary to advance our understanding of the Universe.

In March 2013, the NASA Advisory Council/Science Committee assembled a group of astronomers who would determine the goals and aims of NASA for the next 30 years. The final product is this so-called roadmap officially titled “Enduring Quests Daring Visions — NASA Astrophysics in the Next Three Decades.”

The roadmap first notes three defining questions NASA should continue to pursue:
— Are we alone?
— How did we get here?
— How does the Universe work?

“Seeking answers to these age-old questions are enduring quests of humankind,” the roadmap states. “The coming decades will see giant strides forward in finding earthlike habitable worlds, in understanding the history of star and galaxy formation and evolution, and in teasing out the fundamental physics of the cosmos.”

In order to better address these questions, the roadmap defines three broad categories of time: the Near-Term Era, defined by missions that are currently flying or planned for this coming decade, the Formative Era, defined by missions that are designed and built in the 2020s, and the Visionary Era, defined by advanced missions for the 2030s and beyond.

A schematic of the next 30 years subdivided into three decades across the entire electromagnetic spectrum. Image Credit: NASA 2014

Are we alone?

The Near-Term Era’s goal is to develop a comprehensive understanding of the demographics of planetary systems. The Kepler mission has already supplied a plethora of information on hot planets orbiting close to their parent stars. The WFIRST-AFTA mission — a wide-field infrared survey planned to launch in 2024 — will compliment this by supplying information on cold and free-floating planets.

The Formative Era’s goal is to characterize the surfaces and atmospheres of nearby stars. This will allow us to move beyond characterizing planets as Earth-like in mass and radius to truly being Earth-like in planetary and atmospheric composition. A proposed mission that allows a large star-planet contrast will directly measure oxygen, water vapor, and other molecules in the atmospheres of Earth-like exoplanets.

The Visionary Era’s goal is to produce the first resolved images of Earth-like planets around other stars. The roadmap team hopes to identify continents and oceans on distant worlds using optical telescopes orbiting hundreds of kilometers apart.

How did we get here?

The Near-Term Era will use the James Webb Space Telescope to supply unprecedented views of protostars and star clusters. It will resolve nearby stellar nurseries and take a closer look at the earliest galaxies.

The Formative Era will trace the origins of planets, stars and galaxies across a spectrum of wavelengths. An infrared surveyor will resolve protoplanetary disks while an X-ray surveyor will observe supernova remnants and trace how these incredible explosions affected the evolution of galaxies. Gravitational wave detectors will untangle the complicated dance between galaxies and the supermassive black holes at their centers.

The Visionary Era will peer nearly 14 billion years into the past when ultraviolet photons from the first generation of stars and black holes flooded spaced with enough energy to free electrons. The James Webb Space Telescope will provide an extraordinary means to better view this threshold.

How does the Universe work?

The Universe is full of extremes. Conditions created in the first nanoseconds of cosmic time and near the event horizons of black holes cannot be recreated in the lab. But the Near-Term and Formative Era’s goals will be to measure the cosmos with such precision that scientists can probe the underlying physics of cosmic inflation and determine the exact mechanisms driving today’s accelerating expansion.

The Visionary Era may use gravitational wave detectors to detect space-time ripples produced during the early stages of the Universe or map the shadow cast by a black hole’s event horizon.

The past 30 years have shown a dramatic growth in knowledge with unimaginable turns. Even with such a detailed framework laid out for the next 30 years, it’s likely that many missions are currently beyond the edge of the present imagination. The most exciting results will be drawn from the questions we haven’t even thought to ask yet.

And as with any of the recent “roadmaps” that the various divisions throughout NASA have presented, the biggest question will be if the funding will be available to make these missions a reality.

Again, this 110-page read may be found here.

The roadmap team consists of Chryssa Kouveliotou (NASA/MSFC), Eric Agol (University of Washington), Natalie Batalha (NASA/Ames), Jacob Bean (University of Chicago), Misty Bentz (Georgia State University), Neil Cornish (Montana State University), Alan Dressler (The Observatories of the Carnegie Institution for Science), Scott Gaudi (Ohio State University), Olivier Guyon (University of Arizona/Subaru Telescope), Dieter Hartmann (Clemson University), Enectali Figueroa-Feliciano (MIT), Jason Kalirai (STScI/Johns Hopkins University), Michael Niemack (Cornell University), Feryal Ozel (University of Arizona), Christopher Reynolds (University of Maryland), Aki Roberge (NASA/GSFC), Kartik Sheth (National Radio Astronomy Observatory/University of Virginia), Amber Straughn (NASA/GSFC), David Weinberg (Ohio State University), Jonas Zmuidzinas (Caltech/JPL), Brad Peterson (Ohio State University) and Joan Centrella (NASA Headquarters).

May 21, 2012December 23, 2015 by Jason Major

Dark Matter Makes a Comeback

The Milky Way an moonrise over ESO's Paranal observatory (ESO/H.H. Heyer)

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Recent reports of dark matter’s demise may be greatly exaggerated, according to a new paper from researchers at the Institute for Advanced Study.

Astronomers with the European Southern Observatory announced in April a surprising lack of dark matter in the galaxy within the vicinity of our solar system.

The ESO team, led by Christian Moni Bidin of the Universidad de Concepción in Chile, mapped over 400 stars near our Sun, spanning a region approximately 13,000 light-years in radius. Their report identified a quantity of material that matched what could be directly observed: stars, gas, and dust… but no dark matter.

“Our calculations show that it should have shown up very clearly in our measurements,” Bidin had stated, “but it was just not there!”

But other scientists were not so sure about some assumptions the ESO team had based their calculations upon.

Researchers Jo Bovy and Scott Tremaine from the Institute for Advanced Study in Princeton, NJ, have submitted a paper claiming that the results reported by Moni Biden et al are “incorrect”, and based on an “invalid assumption” of the motions of stars within — and above — the plane of the galaxy.

(Read: Astronomers Witness a Web of Dark Matter)

“The main error is that they assume that the mean azimuthal (or rotational) velocity of their tracer population is independent of Galactocentric cylindrical radius at all heights,” Bovy and Tremaine state in their paper. “This assumption is not supported by the data, which instead imply only that the circular speed is independent of radius in the mid-plane.”

The researchers point out the stars within the local neighborhood move slower than the average velocity assumed by the ESO team, in a behavior called asymmetric drift. This lag varies with a cluster’s position within the galaxy, but, according to Bovy and Tremaine, “this variation cannot be measured for the sample [used by Moni Biden’s team] as the data do not span a large enough range.”

When the IAS researchers took Moni Biden’s observations but replaced the ESO team’s “invalid” assumptions on star movement within and above the galactic plane with their own “data-driven” ones, the dark matter reappeared.

eso1217b — Artist's impression of dark matter surrounding the Milky Way. (ESO/L. Calçada)

“Our analysis shows that the locally measured density of dark matter is consistent with that extrapolated from halo models constrained at Galactocentric distances,” Bovy and Tremaine report.

As such, the dark matter that was thought to be there, is there. (According to the math, that is.)

And, the two researchers add, it’s not only there but it’s there in denser amounts than average — at least in the area around our Sun.

“The halo density at the Sun, which is the relevant quantity for direct dark matter detection experiments, is likely to be larger because of gravitational focusing by the disk,” Bovy and Tremaine note.

When they factored in their data-driven calculations on stellar velocities and the movement of the halo of non-baryonic material that is thought to envelop the Milky Way, they found that “the dark matter density in the mid-plane is enhanced… by about 20%.”

So rather than a “serious blow” to the existence of dark matter, the findings by Bovy and Tremaine — as well as Moni Biden and his team — may have not only found dark matter, but given us 20% more!

Now that’s a good value.

Read the IAS team’s full paper here.

(Tip of the non-baryonic hat to Christopher Savage, post-doctorate researcher at the Oskar Klein Centre for Cosmoparticle Physics at Stockholm University for the heads up on the paper.)

April 3, 2012December 23, 2015 by Jason Major

Finding Out What Dark Matter Is – And Isn’t

This dwarf spheroidal galaxy is a satellite of our Milky Way and is one of 10 used in Fermi's dark matter search. (Credit: ESO/Digital Sky Survey 2)

Astronomers using NASA’s Fermi Gamma-Ray Space Telescope have been looking for evidence of suspected types of dark matter particles within faint dwarf galaxies near the Milky Way — relatively “boring” galaxies that have little activity but are known to contain large amounts of dark matter. The results?

These aren’t the particles we’re looking for.

80% of the material in the physical Universe is thought to be made of dark matter — matter that has mass and gravity but does not emit electromagnetic energy (and is thus invisible). Its gravitational effects can be seen, particularly in clouds surrounding galaxies where it is suspected to reside in large amounts. Dark matter can affect the motions of stars, galaxies and even entire clusters of galaxies… but when it all comes down to it, scientists still don’t really know exactly what dark matter is.

Possible candidates for dark matter are subatomic particles called WIMPs (Weakly Interacting Massive Particles). WIMPs don’t absorb or emit light and don’t interact with other particles, but whenever they interact with each other they annihilate and emit gamma rays.

If dark matter is composed of WIMPs, and the dwarf galaxies orbiting the Milky Way do contain large amounts of dark matter, then any gamma rays the WIMPs might emit could be detected by NASA’s Fermi Gamma-Ray Space Telescope.

After all, that’s what Fermi does.

Ten such galaxies — called dwarf spheroids — were observed by Fermi’s Large-Area Telescope (LAT) over a two-year period. The international team saw no gamma rays within the range expected from annihilating WIMPs were discovered, thus narrowing down the possibilities of what dark matter is.

“In effect, the Fermi LAT analysis compresses the theoretical box where these particles can hide,” said Jennifer Siegal-Gaskins, a physicist at the California Institute of Technology in Pasadena and a member of the Fermi LAT Collaboration.

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So rather than a “failed experiment”, such non-detection means that for the first time researchers can be scientifically sure that WIMP candidates within a specific range of masses and interaction rates cannot be dark matter.

(Sometimes science is about knowing what not to look for.)

A paper detailing the team’s results appeared in the Dec. 9, 2011, issue of Physical Review Letters. Read more on the Fermi mission page here.

March 24, 2012December 23, 2015 by Steve Nerlich

Journal Club – Black Holes Made All The Difference

Today's Journal Club is about a new addition to the Standard Model of fundamental particles.

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According to Wikipedia, a journal club is a group of individuals who meet regularly to critically evaluate recent articles in the scientific literature. And of course, the first rule of Journal Club is… don’t talk about Journal Club.

So, without further ado – today’s article is about how turning complex theory into plain English can lead to advances in science.

Today’s article:
Schutz, B. Thoughts about a conceptual framework for relativistic gravity.

This article is a bit on the philosophical side and involves some debatable historical interpretation. For example, it is claimed that Einstein’s general relativity theory, after an initial buzz in the 1920s, sat in the obscurity of backroom physics through the 1930s and up to the mid 1950s. Indeed, as an example of the maxim that you often have to wait for someone to die before the science can move on, it is claimed that only after Einstein’s death in 1955 did something of a revival take place, which then brought relativity physics back into the mainstream.

The author Bernard Schutz can claim some authority here since his thesis supervisor was Kip Thorne whose thesis supervisor was John A Wheeler. Wheeler, quoting from his Wikipedia write up was an American theoretical physicist who was largely responsible for reviving interest in general relativity in the United States after World War II. And according to Kip Thorne’s Wikipedia write up, Thorne is one of the world’s leading experts on the astrophysical implications of Einstein’s general theory of relativity. Bernard F Schutz’s Wikipedia write up just says he is an American physicist, but give him time.

In the article, Einstein is claimed to be partly responsible for keeping general relativity in the boondocks by dismissing some of its more exciting implications such as black holes and gravitational waves. Instead Einstein doggedly pursued his idea of a unified field theory which led relativity science to an apparent dead end.

Wheeler was at Princeton University at the same time as Einstein and is described as a ‘late collaborator’, although much of his earlier work was in quantum physics and he was closely involved in the Manhattan project.

But Wheeler’s later work and teaching was very focused on the implications of the curvaceous space-time geometry of general relativity, which he communicated via plain English heuristic explanations of some of the wilder implications of that geometry. For example, he was responsible for coining the term black hole as well as the term worm hole. And suddenly general relativity got sexy again. There was an explosion of papers from the 1960s on into the 1990s seeking to grapple with the concept of a black hole – which then reached a fever pitch as astronomical evidence of the existence of black holes began to come in.

Schutz’s essential hypothesis is that it was physicists schooled in quantum mechanics taking a fresh look at relativity theory that made the difference. These were physicists schooled in the approach of we have the math, but what does it mean? Suddenly people like Wheeler were back engaging with Einstein-like Gedanken (thought) experiments. This turned the math into plain-English so that non-relativist physicists suddenly got what it was about – and wanted a piece of the action.

So… comments? Was Einstein inadvertently responsible for delaying the incorporation of relativity into mainstream physics? Or is this article just about a bunch of quantum physicists trying to stake a claim in the development of ‘the other side’ of physics? It’s a story of rivalry, jealousy and curvaceous sexiness – I welcome suggestions about an even more controversial article for the next edition of Journal Club.