Universe Today | Steve Nerlich | 2012 Jan 14
Today’s journal article on the dissection table is about using our limited understanding of dark matter to attempt visualise the cosmic web of the very early universe.
So… dark matter, pretty strange stuff huh? You can’t see it – which presumably means it’s transparent. Indeed it seems to be incapable of absorbing or otherwise interacting with light of any wavelength. So dark matter’s presence in the early universe should make it readily distinguishable from conventional matter – which does interact with light and so would have been heated, ionised and pushed around by the radiation pressure of the first stars.[attachment=0]figure1.jpg[/attachment][/i][/b]
This fundemental difference may lead to a way to visualise the early universe. To recap those early years, first there was the Big Bang, then three minutes later the first hydrogen nuclei formed, then 380,000 years later the first stable atoms formed. What follows from there is the so-called dark ages – until the first stars began to form from the clumping of cooled hydrogen. And according the current standard model of Lambda Cold Dark Matter – this clumping primarily took place within gravity wells created by cold (i.e. static) dark matter.
This period is what is known as the reionization era, since the radiation of these first stars reheated the interstellar hydrogen medium and hence re-ionized it (back into a collection of H+ ions and unbound electrons).
While this is all well established cosmological lore – it is also the case that the radiation of the first stars would have applied a substantial radiation pressure on that early dense interstellar medium.
So, the early interstellar medium would not only be expanding due to the expansion of the universe, but also it would be being pushed outwards by the radiation of the first stars – meaning that there should be a relative velocity difference between the interstellar medium and the dark matter of the early universe – since the dark matter would be immune to any radiation pressure effects.
To visualize this relative velocity difference, we can look for hydrogen emissions, which are 21 cm wavelength light – unless further red-shifted, but in any case these signals are well into the radio spectrum. Radio astronomy observations at these wavelengths offer a window to enable observation of the distribution of the very first stars and galaxies – since these are the source of the first ionising radiation that differentiates the dark matter scaffolding (i.e. the gravity wells that support star and galaxy formation) from the remaining reionized interstellar medium. And so you get the first signs of the cosmic web when the universe was only 200 million years old.
Higher resolution views of this early cosmic web of primeval stars, galaxies and galactic clusters are becoming visible through high resolution radio astronomy instruments such as LOFAR – and hopefully one day in the not-too-distant future, the Square Kilometre Array – which will enable visualisation of the early universe in unprecedented detail.
So – comments? Does this fascinating observation of 21cm line absorption lines somehow lack the punch of a pretty Hubble Space Telescope image? Is radio astronomy just not sexy?
First Map of Universe's Earliest Stars Unveiled
Technology Review | The Physics arXiv Blog | kfc | 2012 Jan 09
The new map shows how the universe might have looked when it was just 30 million years old.
The evolution of galaxies is one of the the great outstanding mysteries of astrophysics. And in recent years, astronomers have taken great strides in tackling the problem.[attachment=1]figure2.jpg[/attachment][/i][/b]
The latest generation of telescopes peer back in time to within a few hundred million years of creation. They clearly show the first galaxies shining brightly only 600 million years after the Big Bang. These galaxies form clusters which themselves stretch out across the cosmos in a vast filamentary-type structure known as the cosmic web.
This structure corresponds more or less exactly to the differences in the density of matter that must have arisen in the instants after creation. Cosmologists think they understand this structure well and have accurately simulated how it came into being.
The only wrinkle in their models is the stars from which galaxies are made, which must obviously have formed earlier.
Astronomers think that earliest stars must have switched on about 30 million years after the Big Bang. And that raises an interesting question: how were these stars distributed through the universe at that time?
This problem is not as straightforward as it sounds. It's easy to imagine that since the galaxies formed from the coalescence of stars, their distributions must be similar.
But astronomers have recently discovered a problem with this line of thinking. Galaxies appear to have formed around massive haloes of dark matter.
But that can't be the case for stars. In the early universe, visible matter would have been accelerated by radiation pressure while dark matter was not. So the earliest stars must have formed from stuff that was moving too quickly relative to the dark matter background to be captured by it. And that means the distribution of the first stars would have been significantly different from the later distribution of galaxies.
Now Eli Visbal at Harvard University and a few pals have performed the first detailed simulation of this effect to create map of the earliest stars. This map of the universe when it was just 30 million years old shows that the universe's earliest occupants must also have formed a cosmic web of their own, albeit different in structure from the one we see today.
Although this is only a simulation, we're likely to find out soon how good it is. The first stars produced light that we ought to be able to see today and a global effort to spot it is currently underway.
Astronomers have built a number of telescopes capable of seeing this light, which is now redshifted into low-frequency radio waves. Observatories such as the Murchison Widefield Array, the Low Frequency Array for radio astronomy (LOFAR) and the Giant Metrewave Radio Telescope will look for light from the oldest stars and eventually show us how accurate this type of simulation can be. We'll see the results in the next few years.
The Grand Cosmic Web of the First Stars - Eli Visbal et al
- arXiv.org > astro-ph > arXiv:1201.1005 > 04 Jan 2012
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