Space: Dark Matter Stars Could Solve Cosmic Mystery

[bystander]

Dark Matter Stars Could Solve Cosmic Mystery
Space.com | Science | 11 Oct 2010

In the early universe, the first stars may have been made not of regular matter, but of its mysterious cousin, dark matter. But exactly how it all happened remains a mystery, and figuring it out could help astronomers understand dark matter itself.

Dark matter is the pesky substance thought to permeate the universe that stubbornly refuses to show itself to telescopes or any other direct detection method scientists can throw at it. Yet researchers can sense it lurking by the gravitational pull dark matter exerts on normal stars and galaxies.

There are many competing theories about what dark matter is actually made of, and each suggestion comes with slightly different properties and implications. Now, a new study calculated which possible forms of dark matter could lead to so-called dark stars.

Not really dark

"Not all kinds of dark matter would be able to form dark stars," said study leader Paolo Gondolo of the University of Utah. "In this sense dark stars are a tool to understand the nature of dark matter." [Video: Dark Matter in 3-D]

The term dark star is somewhat misleading, he said, because in fact these stars would emit light and would be visible. But the matter that reacts in the star's core to form that light would be dark matter, not regular matter.

Though dark stars have not yet been observed by telescopes, some experts think future observatories like the James Webb Space Telescope, a successor to the Hubble Space Telescope, set to launch in 2014, could be up to the job.

"If we detect evidence of dark stars, or if we can say that there are no dark stars, then they present constraints" on what dark matter is made of, Gondolo told SPACE.com.

What makes a star?

Many dark matter candidate particles are their own antimatter partner, which means if two particles come close enough together, they will annihilate to produce energy, some of which is emitted in the form of light.

Today, with the universe spread out, matter is not packed tightly enough for dark matter particles to be condensed to the point of annihilating. But after the Big Bang when the universe was young and comparatively small, the conditions were right for dark stars.

Dark matter would only form a very small fraction of the total mass of such stars – the rest would be normal matter. But the dark matter annihilation process is very efficient, because colliding dark particles would convert all of their mass to energy via Einstein's equation, E=mc². Thus dark stars would shine quite bright.

Eventually, when the dark matter supply inside the star was exhausted, normal matter would condense to begin the regular process of nuclear fusion that fuels most stars. Some of these could still be around today.

Dark matter that can form dark stars - P Gondolo et al

• Journal of Cosmology and Astroparticle Physics 2010(07) 026 (27 July 2010) DOI: 10.1088/1475-7516/2010/07/026
  arXiv.org > astro-ph > arXiv:1004.1258 > 08 Apr 2010 (v1), 09 Apr 2010 (v2)

Were the First Stars Dark
University of Utah | 03 Dec 2007

[bystander]

Cold, Dead Stars Could Help Limit Dark Matter
Wired Science | 15 Oct 2010

Artist’s impression of a neutron star with a powerful magnetic field, called a magnetar. [i](Credit: NASA)[/i]

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Hunting for cold stellar corpses near the center of the galaxy or in star clusters could put new limits on the properties of dark matter.

“You can exclude a big class of theories that the experiments cannot exclude just by observing the temperature of a neutron star,” said physicist Chris Kouvaris of the University of Southern Denmark, lead author of a paper in the Sept. 28 Physical Review D. “Maybe by observations, which come cheaper than expensive experiments, we might get some clues about dark matter.”

Dark matter is the irritatingly invisible stuff that makes up some 23 percent of the universe, but makes itself known only through its gravitational tug on ordinary matter.

There are several competing theories about what dark matter actually is, but one of the most widely pursued is a hypothetical weakly-interacting massive particle (WIMP). Physicists in search of WIMPs have placed experimental detectors deep underground in mines and mountains, and are waiting for a dark matter particle to hit them.

Others have proposed looking for the buildup of dark matter in stars like the sun or white dwarfs. But both subterranean and stellar detection strategies will light up only for WIMPs larger than a certain size. That size is miniscule — about a trillionth of a quadrillionth of a square centimeter — but dark matter particles could be smaller still.

One way to rule out such diminutive particles is to look to neutron stars, suggest Kouvaris and coauthor Peter Tinyakov of the Universite Libre de Bruxelles in Belgium.

Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter.

“For their size and their temperature, they have the best efficiency in capturing WIMPs,” Kouvaris said. Particles up to 100 times smaller than the ones underground experiments are sensitive to could still make a noticeable difference to neutron stars.

After the fires of their birth, neutron stars slowly cool over millions of years as they radiate photons. But if WIMPs annihilate each other whenever they meet like a particle of matter meeting a particle of antimatter, as some models suggest they should, dark matter could reheat these cold stars from the inside.

Kouvaris calculated the minimum temperature for a WIMP-burning neutron star, and found it to be about 179,540 degrees Fahrenheit. That’s more than 10 times hotter than the surface of the sun, but more than 100 times cooler than the sun’s fuel-burning interior. It’s also much cooler than any neutron star yet observed.

Dark matter and ordinary matter are thought to clump up in some of the same places, like the center of the galaxy or globular clusters of stars. So Kouvaris and Tinyakov suggest that astronomers try to find a neutron star colder than the minimum temperature in a region with a lot of dark matter floating around.

“If you observe a neutron star with a temp below the one we predict, that excludes a whole class of dark matter candidates,” Kouvaris said. It could mean the WIMPs are extra-small, or that they don’t annihilate when they meet each other — a property of WIMPs that experiments can’t get at.

Can neutron stars constrain dark matter? - C Kouvaris, P Tinyakov

• Physical Review D 82 063531 (28 Sept 2010) DOI: 10.1103/PhysRevD.82.063531
  arXiv.org > astro-ph > arXiv:1004.0586 > 05 Apr 2010

[rstevenson]

... Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter. ...

I don't follow the logic of that. If a particular neutron star is the same mass as Sol, then wouldn't it attract (or not) just as many particles of any kind as Sol? And since dark matter does, apparently, interact gravitationally with baryonic matter, then once attracted it would be just as likely to stay around or within Sol as a neutron star. Or is there perhaps a more abstruse reason why that statement makes sense, that wasn't included in this news release?

Rob

[neufer]

... Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter. ...

I don't follow the logic of that. If a particular neutron star is the same mass as Sol, then wouldn't it attract (or not) just as many particles of any kind as Sol? And since dark matter does, apparently, interact gravitationally with baryonic matter, then once attracted it would be just as likely to stay around or within Sol as a neutron star. Or is there perhaps a more abstruse reason why that statement makes sense, that wasn't included in this news release?

I think the idea is that Sol may well catch as many WIMPS as a neutron star but those captured WIMPS will just bounce around within the whole volume of the sun and so won't be concentrated into a small collision area the size of Manhattan. (And even the solar WIMPS that do collide and heat up the center of Sol will contribute little to the enormous heat of the center of Sol whereas they might be able to contribute noticeably to the lesser heat of a neutron star.)

[rstevenson]

That makes sense Art. Thanks.

Rob

[bystander]

Wouldn't gravity at the surface (and presumably the interior) of a neutron star be considerably greater than that of the sun? Wouldn't you have to take in the difference in radius (density)? You can't just look at them as point sources of gravity.

[Chris Peterson]

Wouldn't gravity at the surface (and presumably the interior) of a neutron star be considerably greater than that of the sun? Wouldn't you have to take in the difference in radius (density)? You can't just look at them as point sources of gravity.

Yes, but if you're talking about "netting" dark matter, you'd not expect much difference. The region where the gravitational forces are very high is very small, so unless the dark matter density was already high, a neutron star shouldn't obviously be a good net.

[bystander]

Theorists seek dark matter in hot neutron stars
ars technica | Chris Lee | updated 17 Dec 2010

Click to view full size image

Dark matter is an enigma wrapped in a conundrum. We have lots of gravitational evidence for the presence of dark matter. In fact, the evidence is from so many different types of observations, and is all so consistent, that very few astronomers or cosmologists appear to doubt that some type of dark matter exists. That is the enigma: it is very likely to exist, but we know very little of the specific details about what exactly exists.

Going further than that has been a problem. The very nature of dark matter, which makes cosmologists so certain of its existence, means it has the very properties that make it so damn hard to find by any means other than gravity—something of a conundrum, really. A recent paper that looks at how dark matter might be detectable in neutron stars inadvertently makes this problem very clear.

Most cosmologists believe that dark matter consists of weakly interacting massive particles (WIMPs). As the name suggests, they are heavy and they only talk to normal matter very rarely. But rarely is not never and, if we can find a place where there is an awful lot of both normal matter and dark matter, we might be able to observe the consequences of the two colliding. There are a fair number of experiments going on that attempt to do this, and they have some tantalizing results. But tantalizing is all they are—nothing that would get you calling your Mom in excitement in the middle of the night.

So, when I stumbled across a paper discussing the effects of dark matter on neutron stars, I was intrigued. The basic idea, it turned out, was that neutron stars have huge densities, so the likelihood of dark matter colliding with normal matter is greater there than in any other objects in the observable universe. If the neutron star happens to be near the galactic center or in a globular cluster, then there should be a lot of WIMPs around to play with.

What should happen is that WIMPs get gravitationally attracted to the neutron star, fall through the crust and, where the density is highest, they start banging into neutrons, lose energy and heat the star up.

There are a couple of key points here, though. WIMPs still don't like playing with normal matter, so it is expected to take millions of years for the WIMPs and the neutron star to reach thermal equilibrium with each other. That means we are looking for old neutron stars. Second, as the WIMPs heat the star up, the chances of WIMPs colliding with neutrons goes down. The reason for this is a bit complicated—basically, the neutrons all want to occupy the lowest energy states possible so, as the WIMPs heat the star, the available states for neutrons to jump to when they collide are at higher and higher energy, reducing the chances they'll make the jump.

I should note that this is one of the joys of neutron stars: these sorts of calculations are possible because, at their most basic, the stars are fairly simple quantum objects. Meaning that some fairly simple calculations should lead to fairly accurate results.

Now, if you were an astronomer, what should you look for? You should observe that some neutron stars that are a bit hotter than expected. And, if we can measure their mass and temperature accurately, we can accurately estimate something called the cross section, which is basically a measure of how likely a WIMP is to collide with a neutron. We can use that information to make it an easier task to look for WIMPs in the fire hose of data coming from the Large Hadron Collider.

You might think that this would be pretty easy: we are looking for old neutron stars that are too hot. Unfortunately, it isn't quite that simple. If normal matter is falling onto the neutron star, it will heat up. If the magnetic field of the neutron star is still relaxing, then that changes the temperature as well. That limits observations to old neutron stars that are near the center of the galaxy, but aren't sucking in normal matter. We also need to observe pulsar activity from the star—it must be very regular, indicating that the magnetic field has already relaxed.

The authors did attempt to find example neutron stars from existing observations, but only came up with two examples that demonstrate that it is going to be very hard to use neutron star temperature as a signature for dark matter. One star is 140 parsecs from Earth—notably not close to the center of the galaxy—and has a temperature of about 105K. That's too high given the expected dark matter concentration there. The other was even closer to Earth and had a lower temperature, but, notably, was still too hot.

What this means is that, surprisingly, real life is kind of complicated, even for neutron stars. These stars obviously have other heating mechanisms that must be taken into account accurately before we can say with any certainty that dark matter caused some of the heating. This is probably possible, but it is going to take a lot more work to develop the models and cross check them against many different observations