jun 212012


Scientists have discovered a new way to detect the first stars when the Universe was in its infancy at a mere 1% of its present age.


The difference between the velocity of dark matter and that of ordinary matter (baryons, i.e., gas consisting of mostly hydrogen and heium). The velocity difference is small in the blue regions and large in the red regions (the relative velocity is given in units of its root-mean-square value). Comparing to the density image, the velocity shows coherent structure on much larger scales. Regions with a large velocity difference have fewer stars, since the gas moves rapidly and is not captured by the gravity of dark matter concentrations (where the gas must accumulate in order to form stars). 

The research is unveiled for the first time by Professor Rennan Barkana from Tel Aviv University at a conference organised by Liverpool John Moores University and the University of Liverpool and sponsored by the Royal Astronomical Society and the Science and Technology Facilities Council, bringing together nearly a hundred astrophysicists from eighteen countries to discuss the latest results on the most distant and powerful explosions in the universe, Gamma Ray Bursts.


The density of matter. Green regions are average, red regions are denser and blue regions are less dense (We show the overdensity, i.e., the density relative to the mean density, in units of the mean density; e.g., 0.5 means a density of 1.5 times the mean). Regions with a higher density than average form more stars, since gravity is enhanced throughout such regions, making it easier to form the dark matter concentrations in which gas collects and forms stars.

Gamma Ray Bursts are brief, unpredictable bursts of radiation that occur anywhere on the sky and which are thought to be associated with the death of a massive star and the formation of a black hole in the early universe.

Until recently, astronomers believed that it was impossible to observe stars when the Universe was so young and just coming out of its so-called dark age – a time when the universe was permeated by hydrogen gas and before any light sources such as stars had switched on. Now, however, scientists have used powerful computer models to show that an expected difference in the speed of gas and dark matter causes the first stars to clump together into a prominent cosmic web.


The density of stars when the Universe was 180 million years old (i.e., at redshift 20). This shows the new prediction, including the effect of density as well as the velocity difference. Comparing to the distribution of stars as affected by density only, the velocity effect produces a more prominent cosmic web, i.e., larger coherent regions that have a low density of stars, separated by ribbons or filaments of high star formation. The colors correspond to the logarithm of the gas fraction in units of its mean value (Here the mean fraction of all the gas which accumulates in star-forming mini-galaxies is 0.12%).

“The discovery of these web-like structures now makes it feasible for radio astronomers to detect the 21-cm wavelength light from the first stars when the Universe was only 200 million years old and still emerging from its dark ages”, said Dr. Barkana.

Professor Carole Mundell, from LJMU’s Astrophysics Research Institute, who is the lead organiser of the conference said, “This result is very exciting because it opens a new window on an era that has always been considered challenging for observers.”


The density of stars when the Universe was 67 million years old (i.e., at redshift 40). This shows the new prediction, including the effect of density as well as the velocity difference. The effect of the velocity is even more striking at this earlier time, so it has a critical effect on our understanding of the environment in which the very first stars formed, but direct observations of such early times are not feasible in the near future. The colors correspond to the logarithm of the gas fraction in units of its mean value (Here the mean fraction of all the gas which accumulates in star-forming mini-galaxies is 1.5e-8).

LJMU scientists from the Astrophysics Research Institute are at the forefront of Gamma Ray research, with the robotic Liverpool Telescope on the Canary island of La Palma having a uniquely powerful capability to react rapidly to notifications from Gamma-Ray detector satellites – such as NASA’s Swift – and catch the optical counterpart and fading afterglow of the explosion.

Professor Carole Mundell said: “Since the launch of NASA’s Swift satellite in 2004, over 700 new gamma ray bursts have been detected out to the edges of the observable universe. Delegates have presented the state-of-the-art in our understanding of black holes and their environments. We have an exciting agenda covering topics such as the very first stars in the Universe, the nature of space-time and the detection of exotic particles.”


The intensity of radio waves from hydrogen atoms at a wavelength of 21 cm, from a cosmic age of 180 million years. This shows the new prediction (including the effect of both velocity and density), in brightness temperature units of millikelvin. The 21-cm intensity mainly measures the gas temperature (though it is also proportional to the gas density). Regions with a high density of stars have had a higher intensity of X-rays, and thus are hotter and emit more intense 21-cm radiation. This image highlights cold spots, since cold gas absorbs particularly strongly. While such maps may actually be measured sometime in the future, more immediately feasible is to measure the statistical properties of the fluctuations, which can be done even in noisy maps. The velocity effect should produce a much stronger fluctuation signal, making it easier to detect, and the signal should carry a signature of the effect of the cosmic microwave background on the gas from when the Universe was less than 400,000 years old

Professor Rennan Barkana’s research results are published on Nature online on Wednesday 20th June and in an upcoming issue.


The gas temperature when the Universe was 180 million years old. This shows the new prediction, including the effect of density as well as the velocity difference. For easy comparison, both cases (with or without the velocity) are normalized so that the mean gas temperature in the Universe equals the cosmic microwave background at the time shown here. Comparing to the other case, the distribution of stars with velocity produces larger coherent warm regions and, especially, larger cold patches in regions with a large velocity difference between the gas and the dark matter. The colors correspond to the logarithm of the gas (kinetic) temperature (in units of the temperature of the cosmic microwave background).

Note: The images are all from a single thin slice of the simulated volume (a different box from the one used in the figures in Nature, with a different set of random initial conditions). Each image shows a region that is 384 comoving Mpc on a side, corresponding to 1.25 billion light years in today’s Universe. The thickness of the slice is 3 comoving Mpc (9.8 million light years). 

Source: the Royal Astronomical Society (RAS) and for the images: WISE Observatory

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