Primordial Black Holes Probably Don’t Pack a Dark Matter Punch

Waiting for the Andromeda galaxy’s stars to twinkle may have extinguished hope for tiny black holes being a significant dark matter candidate

Should a black hole drift in front of a star, it could trigger a microlensing event, so astronomers set out to estimate the number of primordial black holes in Andromeda [Kavli IPMU]

Using the Andromeda galaxy as a huge detector, astronomers have taken a stab at seeing the unseeable — possibly disproving a hypothesis first put forward by the late Stephen Hawking 45 years ago.

According to Hawking’s work, the universe should be filled with black holes that were formed at the beginning of time, when the universe was a chaotic soup of energy just after the Big Bang. Known as “primordial” black holes, these ancient objects are hypothesized to invisibly occupy modern galaxies, including our own, boosting their dark matter mass.

These black holes aren’t the supermassive monsters that lurk in the centers of most galaxies; they’re not even stellar-mass black holes, formed after massive stars go supernova. Primordial black holes are much smaller than that, having leaked most of their mass via Hawking radiation since their formation 13.8 billion years ago. They should, however, still have powerful gravitational effects on the space surrounding them and, in new research published last week in the journal Nature Astronomy, an international team of researchers have leveraged these hypothetical black holes’ space-time-warping powers to reveal their presence.

Or not, as it turns out.

Central to this study is the effect of microlensing. This astronomical method relies on an object passing between us and a distant star. It has been used to great effect when detecting distant exoplanets, or rogue brown dwarfs wandering through interstellar space. Should one of these objects drift directly in front of a star, its gravitational field can create a magnification effect that briefly brightens the star’s light. The gravitational field creates a natural “lens” out of space-time itself, a prediction that arises from Einstein’s general relativity.

The effect of gravitational microlensing on a star in the Andromeda galaxy should a primordial black hole drift in front [Kavli IPMU]

It stands to reason that even though primordial black holes don’t generate any light themselves, if you stare at at entire galaxy for long enough, you should see a lot of twinkling stars, or microlensing events caused by the hypothetical swarm of primordial black holes the galaxy should contain. Count the number of events, and you can take a statistical stab the total number of primordial black holes in a galaxy like Andromeda, thereby providing an estimate as to how much of the universe’s missing dark matter mass is made up from these objects.

Using the power of the Subaru telescope in Hawaii, the researchers put this to the test, capturing 190 consecutive images of Andromeda over seven hours during one night with the observatory’s Hyper Suprime-Cam digital camera. If Hawking’s theory held, the telescope should have recorded approximately 1,000 microlensing events caused by primordial black holes with a mass of less than our moon drifting in front of Andromeda’s stars. Alas, only one microlensing event was detected that night. From this observation campaign alone, the researchers estimate that primordial black holes make up no more than 0.1 percent of the total dark matter mass in our universe.

Although this elegant study doesn’t necessarily disprove the existence of primordial black holes — one single event is interesting, but not compelling — it does put a wrench in the idea that they dominate the mass holed up in dark matter. So, the quest to understand the nature of dark matter grinds on and, with the help of this study, astronomers have now narrowed down the search by removing primordial black holes from the dark matter equation.

Holographic Universe: Fermilab to Probe Smallest Space-Time Scales

Conceptual design of the Fermilab holometer (Fermilab)
Conceptual design of the Fermilab holometer (Fermilab)

During the hunt for the predicted ripples in space-time — known as gravitational waves — physicists stumbled across a rather puzzling phenomenon. Last year, I reported about the findings of scientists using the GEO600 experiment in Germany. Although the hi-tech piece of kit hadn’t turned up evidence for the gravitational waves it was seeking, it did turn up a lot of noise.

Before we can understand what this “noise” is, we need to understand how equipment designed to look for the space-time ripples caused by collisions between black holes and supernova explosions.

Gravitational wave detectors are incredibly sensitive to the tiniest change in distance. For example, the GEO600 experiment can detect a fluctuation of an atomic radius over a distance from the Earth to the Sun. This is achieved by firing a laser down a 600 meter long tube where it is split, reflected and directed into an interferometer. The interferometer can detect the tiny phase shifts in the two beams of light predicted to occur should a gravitational wave pass through our local volume of space. This wave is theorized to slightly change the distance between physical objects. Should GEO600 detect a phase change, it could be indicative of a slight change in distance, thus the passage of a gravitational wave.

While looking out for a gravitational wave signal, scientists at GEO600 noticed something bizarre. There was inexplicable static in the results they were gathering. After canceling out all artificial sources of the noise, they called in the help of Fermilab’s Craig Hogan to see if his expertise of the quantum world help shed light on this anomalous noise. His response was as baffling as it was mind-blowing. “It looks like GEO600 is being buffeted by the microscopic quantum convulsions of space-time,” Hogan said.

Come again?

The signal being detected by GEO600 isn’t a noise source that’s been overlooked, Hogan believes GEO600 is seeing quantum fluctuations in the fabric of space-time itself. This is where things start to get a little freaky.

According to Einstein’s view on the universe, space-time should be smooth and continuous. However, this view may need to be modified as space-time may be composed of quantum “points” if Hogan’s theory is correct. At its finest scale, we should be able to probe down the “Planck length” which measures 10-35 meters. But the GEO600 experiment detected noise at scales of less than 10-15 meters.

As it turns out, Hogan thinks that noise at these scales are caused by a holographic projection from the horizon of our universe. A good analogy is to think about how an image becomes more and more blurry or pixelated the more you zoom in on it. The projection starts off at Planck scale lengths at the Universe’s event horizon, but its projection becomes blurry in our local space-time. This hypothesis comes out of black hole research where the information that falls into a black hole is “encoded” in the black hole’s event horizon. For the holographic universe to hold true, information must be encoded in the outermost reaches of the Universe and it is projected into our 3 dimensional world.

But how can this hypothesis be tested? We need to boost the resolution of a gravitational wave detector-type of kit. Enter the “Holometer.”

Currently under construction in Fermilab, the Holometer (meaning holographic interferometer) will delve deep into this quantum realm at smaller scales than the GEO600 experiment. If Hogan’s idea is correct, the Holometer should detect this quantum noise in the fabric of space-time, throwing our whole perception of the Universe into a spin.

For more on this intriguing experiment, read the Symmety Magazine article “Hogan’s holometer: Testing the hypothesis of a holographic universe.”

Detecting Gravitational Waves on the Cheap

Forget building gravitational wave detectors costing hundreds of millions of dollars (I’m looking at you, LIGO), make use of the most accurate cosmic timekeepers instead and save a bundle.

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a proposal that involves closely monitoring the regular flashes of spinning neutron stars (or pulsars) to detect very slight “shimmers” in their signal. Although the physics is crazy-complex, by tracking these shimmers over a suitably distributed number of pulsars could reveal the passage of gravitational waves.

However, there’s a problem with this plan; pulsars are notoriously tricky stellar objects, as my colleague Jennifer Ouellette points out:

The problem is that you need to closely monitor rapidly-spinning millisecond pulsars, which are (a) tough to find (only 150 have been found over nearly three decades since pulsars were first discovered), and (b) not very plentiful in the part of the night sky of interest to scientists (northern hemisphere). They tend to clump together in globular star clusters, which makes them useless for detecting gravitational waves.

However, according to results announced by the National Radio Astronomy Observatory (NRAO) at this week’s American Astronomical Society (AAS) meeting in Washington D.C., they’ve discovered 17 new pulsars with the help of NASA’s Fermi Gamma-Ray Space Telescope.

In addition to recent Fermi telescope pulsar discoveries, it would appear that the number of potential targets for NANOGrav are increasing, making a stronger case for the 10 year, $65 million project…

You have to wonder whether building the Laser Interferometer Gravitational-Wave Observatory (LIGO) was worth it (but you can’t be too careful, some terrorist organizations might want to use gravitational waves for evil, so it would be good if we detected them first).

Source: Discovery News

Are Wormholes Quantum Vacuum Cleaners?

The wormhole could form shortcuts in space-time (

General relativity and quantum dynamics don’t get along too well.

If you had to compare the two it would be like evaluating the differences between a Mac and a PC; both are well-honed examples of modern computing, but both are hopelessly incompatible. In computing, this isn’t too much of a problem, you either use a PC or a Mac, or you buy both for their individual strengths (and then complain about Microsoft regardless). But in physics, when you’re trying to find a unified theory, the fact that gravity has been outcast from the Standard Model club, tough questions need to be asked. Although there is some hope being generated by superstring theory, quantum gravity has a long way to go before it can be proven (although high energy particle accelerators such as the LHC will be able to help out in that department).

As pointed out by KFC at the Physics ArXiv Blog, “physicists have spent little time bothering to find out” how quantum mechanics operates in a curved space-time as predicted by Einstein’s general relativity. But now, a physicist has done the legwork and imagined what a quantum particle would do when faced with one of the most famous loopholes in space-time; the mouth of a wormhole. And what popped out of the equations? Another curious force called the “quantum anticentrifugal force.”

So, what’s that all about?

Rossen Dandolo from the Universite de Cergy-Pontoise, France, decided to focus on the wormhole as this is the most extreme example of curved space-time there is. Wormholes are used over and over in sci-fi storylines because they are theorized to link two locations in space-time (thereby forming a shortcut), or even two different universes. As this is space-time we’re talking about, there’s also some possibility of using wormholes as passages through time. Although wormholes sound like a whole lot of fun, in practical terms, they won’t be of much use without some exotic energy to hold the throat of the wormhole open.

Dandolo, however, isn’t too interested in traversing these holes in space-time, he is interested in finding out how a particle acts when in the locality of the mouth of a wormhole.

Beginning with some bedrock quantum theory, Dandolo uses the Heisenberg Uncertainty Principal that stipulates that you cannot know a particle’s momentum and location at the same time. So far, so good. Now, looking at a prediction of general relativity, the wormhole will warp space-time to the extreme, stretching the space around the hole. This space-time stretching causes an increase in uncertainty in the location of the particle. As uncertainty in location increases, the uncertainty in momentum decreases. Therefore, the closer you get to the mouth of the wormhole, the momentum, and therefore particle energy, will decrease.

This interaction between the stretching of space-time and quantum properties of the particle has some amazing ramifications. If the particle’s energy deceases the closer it gets to falling into the wormhole, the wormhole is acting as a potential well; particles will move to a location with less energy. Therefore, a new force — combining both quantum dynamics and general relativity — is acting on particles that stray close to the wormhole: an anticentrifugal force.

This makes wormholes particle vacuum cleaners, exerting a space-time curvature effect on the quantum qualities of matter.

General relativity and quantum dynamics might have some stronger ties than we think…

Source: Wormholes Generate New Kind of Quantum Anticentrifugal Force, by KFC on the ArXiv Blog.

Warp Drives and… Black Holes?


Why do all roads seem to lead to black holes? Man made black holes are supposedly going to be produced by the Large Hadron Collider, swallowing Earth (or, at least, a large fraction of Europe), so it seems only logical that something like a warp drive — a technology of the uber-future requiring uber-energies — would also generate a black hole, right?

Yes, we are talking about a vastly theoretical technology, but according to Italian researchers, the spaceship propulsion device popularized by Star Trek could have grave consequences for Planet Earth.

Over the past week, I’ve been deep inside the science behind faster-than-light-speed propulsion and time travel as a part of the Discovery Space Wide Angle: Surfing Spacetime, and I feel well versed in the astounding physics that could make warp speed a possibility in the future. All this started when interviewing one of the leading authorities on warp drive propulsion, Dr Richard Obousy, who is not only upbeat about the possibilities of the futuristic warpship, he’s done the math to prove that a sufficiently advanced civilization could “surf” on a spacetime wave.

However, there’s a catch. Well, two.

Firstly, we need to develop an understanding for dark energy. And second, we need a gargantuan energy source.

Dark energy is a cosmological theory that explains the continued expansion of the universe. This energy pervades all of the cosmos, explaining everything from the grouping of galactic clusters to the faster-than-light-speed inflationary period immediately after the Big Bang. There’s a lot of indirect observation of dark energy and its effects on spacetime. It’s out there, but it’s a tough proposition to think we might be able to harness it someday. But then again, we said that about electricity once, who knows what technological revolutions await us in the decades and centuries ahead.

Assuming we find a way of harnessing dark energy, how can we use it? This is where visionary physicists like Dr Obousy come in. Skipping over the superstring small-print of extra-dimensional theory, we basically need a huge amount of energy to manipulate the universal dark energy, thereby shrinking and expanding vanishingly small dimensions beyond our three dimensional universe.

So how much energy is needed warp spacetime, allowing a futuristic spaceship to zip through space? “Some back of the envelope calculations I performed last year indicated approximately the mass energy contained within the planet Jupiter,” Richard told me.

This sounds like a lot of energy! However, there’s a trend, the rest mass energy of Jupiter is actually an improvement on previous warp drive calculations. “The very early warp drive calculations indicated that one would need more mass energy than was available within the entire universe… that’s TRILLIONS of Jupiters!

This improvement is down to recent developments in superstring theory and quantum dynamics. It would appear that the energy requirements for a warp drive improves with developments in physics. If this trend continues, we may find other energy saving ways to make a warpship a reality.

However, there are some practical issues putting the breaks on travelling at warp speed. Only recently, I reported on a study focusing on quantum fluctuations as the warp “bubble” (containing our warpship) blasts through the light speed barrier: the occupants could get roasted by Hawking Radiation.

Today, another problem has surfaced from the extreme warp equations: black holes (who would have guessed?). Italian physicist Stefano Finazzi of Italy’s International School for Advanced Studies has crunched the numbers and wondered about what would happen when the energy runs out. It’s all very well generating a Jupiter’s rest mass-worth of energy, but how will it be sustained by the warp drive? What will happen when all the energy is depleted?

Eventually the energy would run out. The [warp] bubble would rupture, with catastrophic effects. Inside the bubble the temperature would rise to about 1032 degrees Kelvin, destroying almost anything on the bubble.Eric Bland, Discovery News

It gets better, Finazzi also predicts a fair amount of doom outside the warp bubble, too. “We know that the warp drive will be destabilized,” he added. “But we do not know if it will in the end explode or collapse to a black hole.”

Don’t go running out of gas any where near Earth is all I say

Although these implications of doom and gloom should have given Jean Luc Picard a panic attack whenever he said “engage!” or “make it so Number One…”, we have to remind ourselves warp drive propulsion isn’t even close to being a reality. Dr Obousy and warp scientists before him are only just beginning to assemble a theoretical framework around the sci-fi notion of warping spacetime, so to already be predicting warp speed fail seems a little premature in my opinion.

In response to the Hawking Radiation problem, Dr Obousy pointed out that if we get to the point of generating vast quantities of energy, harnessing the spacetime warping power of dark energy, we should be able to at least have a stab at finding solutions to these potential warp drive problems.

Objections are good, but usually we find smart ways of circumventing problems. Humans are good at that,” he said.

I agree.

Forget Black Holes, Let’s Look For Black Rings

A bubble ring. Could a black hole take on this shape at higher dimensions? (©

Black holes are as extreme as anything can get. When a massive structure can no longer sustain its own gravity, it will collapse to a point known as a singularity. For example, a massive star after it has gone supernova may leave one of these singularities behind, a remnant of massive star death, sucking any local matter into a one-way trip to the guts of space-time.

At a certain point, when light itself succumbs to the black hole’s gravity, an event horizon forms, beyond which universal physics breaks down; we have very little idea about what lies inside the event horizon. All we do know is that you don’t want to fall into one, you’d be stretched and spaghettified. Spaghettification is due to extreme (and when I say extreme, I mean as-extreme-as-it-can-get) tidal forces between your head to your toes.

So, the message is: Don’t play with black holes, it can only end in tears.

Now the Black Hole Health & Safety lecture is over, it’s time to talk about “black rings”. Under certain conditions, black holes may not be the mathematical singularities we once knew and (thought we) understood.

In a recent publication by Masashi Kimura at Osaka City University in Japan, the black ring idea is explored in 5-dimensional space. In the space-time we know and love, there are three spatial dimensions and one temporal dimension. We are four-dimensional creatures. When string theory came along in the 1980’s we really began to appreciate that there could be more than the four dimensions we live in.

Previously, cosmologists have entertained the thought that black rings may exist in our 4D space-time. However, the big problem comes when trying to understand how these structures maintain their shapes; surely they should simply collapse and form your regular black holes? Actually, it depends on how big they are and how the competing forces balance out.

As the Universe is expanding, it is thought black rings could exist if they are of scales similar to the cosmological constant (this constant was derived by Einstein to explain a “flat” Universe, but later it was found the constant was required to characterize the universal expansion as observed by Edwin Hubble in 1929). If a black ring exists in 4D space-time, its gravitational collapse would be countered by the expansion of space-time (as characterized by the cosmological constant).

A bubble ring, as made by a dolphin, for fun (©

The only analogy I can relate this to in the terrestrial world is bubble rings (or, indeed, smoke rings). When under water, a bubble will rise to the surface. However, under the constriction of surface tension, the bubble will form the smallest possible shape. When a bubble ring is produced, there needs to be a balance between surface tension and a vortex. The surface tension pulls in, while the vortex maintains the bubble ring shape, pushing out.

In the case of the black ring, gravity is pulling inward, while the expansion of space-time is countering it, pushing out. In this situation, in an expanding Universe, there could be enduring examples of black rings out there.

In Kimura’s research, not only are black rings a possibility, there could be a number of different complex shapes that could form when considering these extra dimensions. When the Universe was young, multiple interacting black rings may have been possible, eventually coalescing to form black holes.

Although this research is very interesting, it is hard to imagine how we could observe these higher-dimensional black rings. Would we see them as a singularity (i.e. a black hole) in our 4D space-time? Or would they even be unobservable for lower-dimensional beings such as ourselves?

Publication: Dynamical Black Rings with a Positive Cosmological Constant, Masashi Kimura, 2009. arXiv:0904.4311v2 [gr-qc]

Via: arXiv blog

Is the Universe a Holographic Projection?

Luke and Obi-Wan look at a 3D hologram of Leia projected by R2D2 (Star Wars)

Could our cosmos be a projection from the edge of the observable Universe?

Sounds like a silly question, but scientists are seriously taking on this idea. As it happens, a gravitational wave detector in Germany is turning up null results on the gravitational wave detection front (no surprises there), but it may have discovered something even more fundamental than a ripple in space-time. The spurious noise being detected at the GEO600 experiment has foxed physicists for some time. However, a particle physicist from the accelerator facility Fermilab has stepped in with his suspicion that the GEO600 “noise” may not be just annoying static, it might be the quantum structure of space-time itself
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