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380,000 years after the Big Bang, the Universe cooled from being a hot soup of plasma, to a temperature where protons and electrons could combine to form atoms. This calm period of neutral hydrogen in universal history didn’t last for long however. The neutral hydrogen atoms were ripped apart once more, by a mechanism that would go on to reionize the entire Universe, a process that eventually ended a billion years after the Big Bang.
It is thought the first stars that formed prior to the reionisation epoch probably pumped out some fierce ultraviolet radiation, ionizing the neutral hydrogen, but a new (controversial) theory has been put forward. Did dark matter have a role to play in the reionisation the Universe?
As 85% of the Universe is composed of a type of matter we have yet to fully account for, it seems only natural that scientists would be looking into the possibility that dark matter had a role to play soon after the Big Bang. Although scientists are fairly confident that the reionisation period was driven by the emissions from the very first stars, there are some observational factors that could suggest dark matter annihilation might have had a role to play in the evolution of the Universe.
This is according to Dan Hooper and Alexander Belikov from Fermilab in Batavia, Illinois, in any case. In their theory recently published, the researchers examine the physics behind dark matter annihilation as the mechanism that drove the reionisation epoch.
In Hooper and Belikov’s work, they focus on dark matter that is theorized to have clumped together under gravitational attraction as the Universe cooled during the neutral hydrogen era (known as the “Dark Ages” – the Universe would have been opaque due to lack of stars and lack of electromagnetic radiation). When the dark matter during this time clumped, it is predicted to annihilate. During dark matter annihilation, high energy gamma-rays are predicted to be generated. Where gamma-radiation goes, ionization of matter is sure to follow.
“A single gamma ray might reionise 1000 hydrogen atoms,” says Hooper. “The mechanism could easily have reionised the universe.”
By their reasoning, rather than emissions from stars that may have been forming at the start of the reionisation epoch, a far more potent ionization mechanism could have flooded the Universe. However, some scientists are skeptical of this idea.
“We have no evidence yet that any dark matter has ever annihilated,” says Charles Bennett, principal investigator on NASA’s WMAP satellite, which has been studying the reionisation epoch. “I am not saying it is wrong, but it sounds a bit too contrived for me to eagerly accept it.” Bennett sees the dark matter argument as one mystery (reionisation) being explained by another mystery (does dark matter even annihilate?).
For now, the idea that dark matter may have been the underlying mechanism ionizing our Universe remains highly theoretical. But Hooper is eager to study the data from ESA’s upcoming Planck mission as this observatory will be able to study how reionisation proceeded with time. “The time signature of dark matter reionisation will be different from that brought about by stars,” says Hooper.
Source: New Scientist
@solrey: I thought that exact same thing. However, astronomers are saying that Himiko could be an object that has a completely different explanation than what we conventionally believe. The blob could be a dark matter annihilation region… now that would be one hell of a discovery! (But it’s probably just made of stars, interesting ideas though.)
Cheers, Ian 😀
I must admit, I have to join the skepticism here somewhat (which is atypical for me!). Surely if we can account for reionisation without needing a dark matter annhilation mechanism (and, to the best of my knowledge, we can) — then isn’t this theory in violation of Occam’s Razor…?
Ying/yang,light/dark, good/evil… an the Winners is……..yet to be determend
It seems that the information presented in this article contradicts the information from the recent article linked below:
http://www.universetoday.com/2009/04/22/new-mystery-from-cosmic-dawn-the-blob/
No biggie, but the correct spelling is Yin (yin-yang). Ying is a proper name, either a female first name or a familial last name. 🙂
I am somewhat skeptical of this. I have not yet looked further, but I question what the source of DM annihlations is.
Lawrence B. Crowell
I wish I could borrow their time machine to go back in time and observe the origin of the universe so I know what they are talking about. And does anyone have some Dark Matter I could borrow?
The source of DM annihilation?
I suppose if we accept the possibility of dark matter – we have to accept the possibility of dark antimatter.
(I’m not, of course suggesting that the net result is dark energy)
I guess it’s worthwhile going through the motions on this just to take DM theory through to its current logical conclusion. It makes predictions and is observationally differentiable from our current view,
so there is no harm in indulging in such speculation. Soon enough we’ll be able to peer back to that era to check it out. Hopefully.
Who knows – it may even turn out to be on the money, and the reionisation era may end up being an important cosmic laboratory for studying the behaviour of our much hypothesised DM…
Anti-dark matter is of course the obvious answer. Dark matter is potentially the supersymmetric pair of know particle fields, such as the photino, the super pair of the photon. A mixed eigen-state of the super pair of the photon, Higgs, neutrion called the neutralino is the best candidate, since it has the lowest expected mass.
Yet this raises the obvious question. Why did not most DM and anti-DM annihilations occur long before the reionization period? The stuff interacts very weakly, and its analogue of gauge interactions is small. So we might expect that it takes longer for the CP-violating situation to manifest as anti-DM is cancelled out by DM with some residual DM left. But why did it start to take effect 500 billion years after the BB? That is something I find a bit puzzling.
Lawrence B. Crowell
I would offer that we have no evidence yet of dark matter, period. Lot’s of speculation, but no evidence.
And dark matter is a mystery used to explain the mystery of the ‘anomolous’ rotation curves of galaxies, but that’s ok?…
Cheers, Dave.
davesmith_au Says:
April 26th, 2009 at 9:00 pm
“I would offer that we have no evidence yet of dark matter, period. Lot’s of speculation, but no evidence.”
Well, I guess that depends on what you class as evidence. Clearly, we cannot touch or feel it, or even measure it in an Earth-bound lab as of yet. But astronomy is a different science – rarely can we perform such experimentation. Astronomy has more to do with observations of remote phenomena that we have no control over and the subsequent interpretation of said observations in a self-consistent framework. It draws elements and results from other sciences, particularly physics and chemistry, but in the end we are limited by the fact that almost all the stuff we want to study in the universe is beyond our direct reach or control forever. Success in astronomy is a theory that fits all observations, all known laws of science and is self consistent. Progress in astronomy is seeing an effect and proposing a theory to explain it that is testable and falsifiable. So from this point of view, DM does have supporting evidence – it predicts and explains a number of effects and phenomena that have been verified, and does so for that which we cannot otherwise interpret by way of a competing theory that is largely complete, self consistent or fits with other established laws of physics. Further than that, it makes predictions that have not been verified – so indeed there is lots of speculation surrounding the mysterious proposition, but it is all testable in the end. And observation will be brought to bear on it all.
“”“I am not saying it is wrong, but it sounds a bit too contrived for me to eagerly accept it.” Bennett sees the dark matter argument as one mystery (reionisation) being explained by another mystery (does dark matter even annihilate?).””
“And dark matter is a mystery used to explain the mystery of the ‘anomolous’ rotation curves of galaxies, but that’s ok?…”
Agreed – that was quite a silly statement. However, I guess it’s about degrees. Postulating the existence of a form of matter that only interacts via the gravitational and weak forces to explain the anomalous rotation curves of galaxies (plus a great many other phenomena) is one degree of speculation, yet it is almost implied if we are to take current theories as anything approaching correct.
Postulating properties of this postulated matter (such as being able to annihilate) with only very limited evidence to suggest what this stuff might be (let alone whether it exists at all) is another step away on the speculative ladder. As I said before though – all of these speculations make testable predictions as the article makes clear. They cannot remain untested for very long with the current pace of development and discovery in astronomy…
Is the correct terminology ‘ anti dark matter’ – because ‘dark anti matter’ (or ‘dark antimatter) seems more appropriate to me ….
@Dave: you said “I would offer that we have no evidence yet of dark matter, period. Lot’s of speculation, but no evidence.”
Astrofriend makes a good point: the astronomical evidence for dark matter (actually, cold dark matter) is at least as good as that for the existence of lots of other things that have not been created, or even observed, here on Earth yet (and in some cases, very likely never will). Further, CDM is very successful, in the sense that it can explain a wide range of different kinds of observation and can predict the results of many that have not yet been made (but could, and likely will, be).
And there’s more: there are no viable alternative explanations for the relevant astronomical observations, certainly none with comparable explanatory and predictive power (versions of MOND which are consistent with relativity are the closest alternatives I know of, and they are much less powerful).
“And there’s more: there are no viable alternative explanations for the relevant astronomical observations, certainly none with comparable explanatory and predictive power (versions of MOND which are consistent with relativity are the closest alternatives I know of, and they are much less powerful).”
Yes there is a viable, alternative theory. But ya’ll ignore it or scoff at it, hurling insults and ridicule towards those who realize the validity of the theory. And now we’re not even supposed to invoke it’s name here.
There are so many contradictions in the standard model it’s ridiculous. One day it’s dark matter this, dark matter that. The next day it’s modifying gravity theory eliminating the need for dark matter all together. The emperors clothes are frayed beyond recognition at this point.
@Solrey: Don’t waste your time with “OOM” Nereid.
I hope she won’t terrorize this forum the way she made BAUT uninteresting.
@solrey: you said “Yes there is a viable, alternative theory. But ya’ll ignore it or scoff at it, hurling insults and ridicule towards those who realize the validity of the theory. And now we’re not even supposed to invoke it’s name here.”
I’m rather at a loss to know what to say.
MOND, and the various extensions which seek to incorporate relativity, is not ignored, not scoffed at, etc. Far from it; there are dozens and dozens of papers published on these alternatives, every year!
And these are certainly not the only alternatives that you’ll find in the pages of ApJ (Astrophysics Journal), for example, or MNRAS (Monthly Notices of the Royal Astronomical Society), to name just two of peer-reviewed journals (though there are certainly fewer papers on other alternatives, not least because they have many more shortcomings than MOND etc).
Could it be that you are referring to a different alternative? I don’t mean crackpot ideas, of course, but one that is described in at least a handful of papers in relevant journals, within the last decade or so.
You also said: “There are so many contradictions in the standard model it’s ridiculous.”
There are? By “standard model” I assume you mean LCDM cosmological models (do you?); if so, then I don’t know how you came to this conclusion, but it would seem it certainly is not from an understanding of the extraordinary success of such models. May I suggest some reading material for you, that outlines just how successful it is?
Then you said: “One day it’s dark matter this, dark matter that. The next day it’s modifying gravity theory eliminating the need for dark matter all together.”
That’s a pretty good summary of how science works, wouldn’t you say? I mean, no matter what branch of science you’re looking at, today’s theories and explanations will be superceded by others – whether tomorrow, next year, or next millennium – a science where this didn’t happen would be a dead science.
Finally, you write: “The emperors clothes are frayed beyond recognition at this point.”
What on earth do you mean?!?
@ignoramus
I see what you mean. Good advice, thanks.
🙂
The GLAST spacecraft, remaned Fermi and the PAMELA detector have detected an anomalous flux of positrons from the Milky Way center. This is in keeping with neutralino decay channels at the ~ TeV range in energy, or neutralino mass. So we may be gatting particle astrophysical data on CDM.
Lawrence B. Crowell
It s should also be pointed out that certain proposed forms of DM may not decay by annihilation with antiparticles but may decay in other ways (axions, for one come to mind). Also, most papers I’ve seen on DM annihilation focus on ‘leptonic’ decays for certain types of DM. Recently a paper posted at arXiv.org proposed a possible ‘hadronic’ decay mode with antideuterons being one of the decay products, along with positrons. The paper can be found here: http://arxiv.org/PS_cache/arxiv/pdf/0904/0904.1410v2.pdf . The authors do point out several observations that could be performed to test this theory, so, time may tell. At least the current crop of papers shows vigorous research being done in this field.
Whoops, for positrons, read antiprotons. (Possible antiproton excesses have been observed, as well as positron excesses.)
InvaderXan Says:
I must admit, I have to join the skepticism here somewhat (which is atypical for me!). Surely if we can account for reionisation without needing a dark matter annhilation mechanism (and, to the best of my knowledge, we can) — then isn’t this theory in violation of Occam’s Razor…?
*****
Occam’s Razor doesn’t necessarily mean anything. If two competing theories for something have different amounts of assumptions, then generally it is true that the simplest theory is most likely correct. On the other hand, some of those assumptions in the theory with more assumptions could be correct and actually cause that theory to have fewer assumptions than the simpler one (in which case it probably has wrong assumptions). Occam’s Razor is a good place to start for which theory is likelier (assuming that everyone agrees on all of the assumptions) but it doesn’t necessarily mean that one theory is more correct than another. Many theories have become more complex as more knowledge has been learned about a particular subject which would violate Occam’s Razor if you left the new knowledge in the “assumption” category. Only time and further experimentation will decide which is correct.
The neutralino decay is a leptonic channel. I think the idea for hadronic decay is that the CP violating process in leptonic channels is compensated for by the axion. The axion has not been expeirmentally found, though its tiny mass makes it tough to find.
It should be interesting to see how the DM problem works through in the near future.
@ Lawrence B. Crowell: A different group of researchers posted their theoretical work on axions the same day as the previous paper. In “Axions as Dark Matter Particles” and described their ‘Axion Dark Matter eXperiment’ ADMX to look for axions. Their paper is here: http://arxiv.org/PS_cache/arxiv/pdf/0904/0904.3346v1.pdf . While the hunt for axions (or their absence) has been a long one, I think it illustrates just how science is done via theory and experimentation. “It should be interesting to see how the DM problem works through in the near future.” (btw, I’m hoping neutralinos pan out)
solrey Says:
“There are so many contradictions in the standard model it’s ridiculous. One day it’s dark matter this, dark matter that. The next day it’s modifying gravity theory eliminating the need for dark matter all together. The emperors clothes are frayed beyond recognition at this point.”
Yes so what? This is how science works, theories are created, and the get tested! If a theory fails in it’s test by other cross references using different techniques and means then it just gets disregarded and thrown in the bin.
Sometimes we get conflicting theories so they will be tested both of them by independend researches and those that fails tests are bad. It could be perfectly possible that both theories are dead wrong!
You can only test theories by throwing it to the lions (yes other science peorple) trying to schread it to peaces. If it gets schreadded then it did not pass a test. Most alternative theories just gets schredded in the first pass.
The only reason why some alternative theories gets beeing popped up after beeing schreaded by scientists is because om some theories have become a real religion and some guru’s are trying to impress people with secret knowledge that now one knows so they get these brainless followers that have no clue in understanding what this guru means but it sounds impressive so it must be true.