Starship Asterisk*

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.

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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.

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?

The new map shows how the universe might have looked when it was just 30 million years old.

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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.

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.

• arXiv.org > astro-ph > arXiv:1201.1005 > 04 Jan 2012

Ann wrote:Very interesting, but I don't understand the images. Two sets of images are set against each other. One set of images shows the clumpiness of the dark matter distribution and the proto-galaxy distribution, and the other set shows the clumpiness of the stellar distribution.

But which is which?

Ann

TAU uses radio waves to uncover oldest galaxies yet

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Windows to the past, stars can unveil the history of our universe, currently estimated to be 14 billion years old. The farther away the star, the older it is — and the oldest stars are the most difficult to detect. Current telescopes can only see galaxies about 700 million years old, and only when the galaxy is unusually large or as the result of a big event like a stellar explosion.

Now, an international team of scientists led by researchers at Tel Aviv University have developed a method for detecting galaxies of stars that formed when the universe was in its infancy, during the first 180 million years of its existence. The method is able to observe stars that were previously believed too old to find, says Prof. Rennan Barkana of TAU's School of Physics and Astronomy.

Published in the journal Nature, the researchers' method uses radio telescopes to seek out radio waves emitted by hydrogen atoms, which were abundant in the early days of the universe. Emitting waves measuring about eight inches (21 centimeters) long, the atoms reflect the radiation of the stars, making their emission detectable by radio telescopes, explains Prof. Barkana. This development opens the way to learning more about the universe's oldest galaxies.

Reading signals from the past

According to Prof. Barkana, these waves show a specific pattern in the sky, a clear signature of the early galaxies, which were one-millionth the size of galaxies today. Differences in the motion of dark matter and gas from the early period of the universe, which affect the formation of stars, produce a specific fluctuation pattern that makes it much easier to distinguish these early waves from bright local radio emissions.

The intensity of waves from this early era depends on the temperature of the gas, allowing researchers to begin to piece together a rough map of the galaxies in an area of the sky. If the gas is very hot, it means that there are many stars there; if cooler, there are fewer stars, explains Prof. Barkana.

These initial steps into the mysterious origins of the universe will allow radio astronomers to reconstruct for the first time what the early universe looked like, specifically in terms of the distribution of stars and galaxies across the sky, he believes.

A new era

This field of astronomical research, now being called "21-centimeter cosmology", is just getting underway. Five different international collaborations are building radio telescopes to detect these types of emissions, currently focusing on the era around 500 million years after the Big Bang. Equipment can also be specifically designed for detecting signals from the earlier eras, says Prof. Barkana. He hopes that this area of research will illuminate the enigmatic period between the birth of the universe and modern times, and allow for the opportunity to test predictions about the early days of the universe.

"We know a lot about the pristine universe, and we know a lot about the universe today. There is an unknown era in between when there was hot gas and the first formation of stars. Now, we are going into this era and into the unknown," says Prof. Barkana. He expects surprises along the way, for example involving the properties of early stars, and that observations will reveal a more complicated cosmological reality than was predicted by their models.

• Nature 487(7405) 70 (05 July 2012) DOI: 10.1038/nature11177
  arXiv.org > astro-ph > arXiv:1201.1005 > 04 Jan 2012 (v1), 26 Jun 2012 (v3)