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Tag Archives: early universe

Dark Ages, Dark Matter

Cosmologists call the first couple of hundred million years of the universe’s history the Dark Ages. This is the period until the first stars formed. The Cosmic Dawn is the name given to the epoch during which these first stars formed.

Now there has been a stunning detection of the 21 centimeter line from neutral hydrogen gas in that era. Because the first stars are beginning to form, their radiation induces the hyperfine transition for electrons in the ground state orbitals of hydrogen. This radiation undergoes a cosmological expansion of around a factor of 18 since the era of the Cosmic Dawn. By the time it reaches us, instead of being at the laboratory frequency of 1420 MHz, it is at around 78 MHz.

This is a difficult frequency at which to observe, since the region of spectrum is between the TV and FM bands in the U.S. and instrumentation itself is a source of radio noise. Very remote, radio quiet, sites are necessary to minimize interference from terrestrial sources, and the signal must be picked out from a much stronger cosmic background.

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Image credit: CSIRO-Australia and EDGES collaboration, MIT and Arizona State University. EDGES is funded by the National Science Foundation.

This detection was made in Western Australia with a radio detector known as EDGES, that is sensitive in the 50 to 100 MHz range. It is surprisingly small, roughly the size of a large desk. The EDGES program is a collaboration between MIT and Arizona State University.

The researchers detected an absorption feature beginning at 78 MHz, corresponding to a redshift of 17.2 (1420/78 = 18.2 = 1 + z, where z is redshift) and for  the canonical cosmological model it corresponds to an age of the universe of 180 million years.

The absorption feature is much stronger than expected from models, implying a lower gas temperature than expected.

At that redshift the cosmic microwave background temperature is at 50 Kelvins (at the present era it is only 2.7 Kelvins). The neutral hydrogen feature is seen in absorption against the warmer cosmic microwave background, and is much cooler (both its ‘spin’ and ‘kinetic’ temperatures).

This neutral hydrogen appears to be at only 3 Kelvins. Existing models had the expectation that it would be at around 7 Kelvins or even higher. (A Kelvin degree equals a Celsius degree, but has its zero point at absolute zero rather than water’s freezing temperature).

In a companion paper, it has been proposed that interactions with dark matter kept the hydrogen gas cooler than expected. This would require an interaction cross section between dark matter and ordinary matter (non- gravitational interaction, perhaps due to the weak force) and low velocities and low masses for dark matter particles. The mass should be only a few GeV (a proton rest mass is .94 GeV). Most WIMP searches in Earth-based labs have been above 10 GeV.

These results need to be confirmed by other experiments. And the dark matter explanation is speculative. But the door has been opened for Cosmic Dawn observations of neutral hydrogen as a new way to hunt for dark matter.

References:

“A Surprising Chill before the Cosmic Dawn” https://www.nature.com/articles/d41586-018-02310-9

EDGES science: http://loco.lab.asu.edu/edges/edges-science/

EDGES array and program: https://www.haystack.mit.edu/ast/arrays/Edges/

R. Barkana 2018, “Possible Interactions between Baryons and Dark Matter Particles Revealed by the First Stars” http://www.nature.com/articles/nature25791

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Blue Tides and the Milky Way

I recently wrote about the  largest high-redshift cosmological simulation of galaxy formation ever, which has been recently completed by a group of astrophysicists in the U.S. and the U.K. This tour-de-force simulation, named BlueTides, was performed on the Blue Waters Cray XE supercomputer at NCSA and employed 648,000 cores. The researchers utilized approximately 700 billion particles (!) to represent dark matter and ordinary matter and to create galaxies inside the supercomputer.

You can find the full article describing the simulation at insidehpc.com.

Galaxies are the fundamental building blocks of the large scale structure of the universe. Very early on, before the first galaxies formed, the universe was a highly uniform mix of dark matter and ordinary matter, but with about 5 times as much dark matter by mass relative to the ordinary matter (protons, neutrons, electrons) that makes up the visible parts of galaxies, including stars, gas and dust. Ares of higher dark matter density play a key role in gravitationally attracting ordinary matter that forms galaxies and stars.

When we think of the word galaxy we typically think of beautiful modern day spiral galaxies, such as the Andromeda Galaxy. Spiral galaxies are flattened, rotating disks; the spiral arms represent concentrations of matter and of star-forming regions. The most distant disk-shaped galaxies that have been detected are at redshifts of 2 to 3, so we are seeing them as they were when the universe was around 2 to 3 billion years old. (The higher the redshift the more distant the galaxy and also the farther back in time we are looking, toward the universe’s origin some 13.8 billion years ago).

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Two spiral galaxies starting to collide. Image Credit: Debra Meloy Elmegreen (Vassar College) et al.,
& the Hubble Heritage Team (AURA/STScI/NASA)

The BlueTides simulation provides insight into what was going on when the universe was only around 1/2 a billion years old, with galaxy redshifts around 8 to 10. It does a good job of matching the limited observational data we have at such highredshifts, in particular the rest frame (before redshift) ultraviolet luminosities of the earliest detected galaxies from the Hubble Space Telescope surveys.

Their simulation finds that, among the most massive galaxies in their simulation, “a significant fraction are visually disk-like, and surprisingly regular in shape”. In other words, they appear to be the progenitors of present-day spiral galaxies. They find that, at a redshift of 8, a full 70% of their virtual galaxies with masses above 10 billion solar masses are classifiable as disks, since a majority of stellar orbits lie in the plane of the disk.

Fig1.Fengetal.milkyway

Simulated high-redshift galaxies from BlueTides – Figure 1 from Feng et al.

They also find that mergers are not of major significance in the build-up of these early massive galaxies. Rather it appears that they grew primarily by cold gas arriving from preferred directions, namely along filaments in the density distribution of the background gas. It is well known that the universe has a web-like or filamentary structure of high density regions interspersed with voids (relatively empty regions). This filamentary structure is believed on the basis of many simulations of the large-scale universe to have begun at an early date.

A future infrared satellite known as WFIRST will have a field of view 200 times larger than the Wide Field Camera on the Hubble. Also, its design for infrared radiation detection makes it appropriate for studying the light from high-redshift galaxies. The authors predict that a survey of 2000 square degrees with WFIRST should find roughly 8000 massive disk-type galaxies at redshifts above 8. Future very large ground-based telescopes will be able to make follow-up observations of galaxies discovered by WFIRST. Such observations will provide further insight into the nature of galaxy formation, including accretion of material from the background and the details of dark matter’s role in the process.

Reference

Feng et al. 2015, “The Formation of Milky Way-Mass Disk Galaxies in the First 500 Million Years of a Cold Dark Matter Universe” http://arxiv.org/abs/1504.06618