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On February 28th, the EDGES team led by Judd Bowman of Arizona State University claimed evidence of the first stars in our universe — stars born 13.7 billion years ago, about 100 million years after the Big Bang.
Before EDGES, the oldest detected stars and galaxies ever observed are in this Hubble Space Telescope image.
Hubble Ultra Deep Field Image
To understand what this means, let’s start from:
In the Beginning…
Our universe was born 13.82 billion (plus or minus 48 million) years ago. It began very small and very hot — probably smaller than a trillionth of a trillionth of the size of the smallest atom that exists today, and possibly hotter than 100 million, trillion, trillion degrees. It rapidly expanded in size, and as it did, its temperature plummeted.
By the time our universe was 1 second old, its temperature was “only” 10 billion degrees, and almost all the particles that exist today had already been created.
At age 379,000 years, the temperature was 3000 K (4900ºF), cold enough for protons and electrons to stick together forming neutral hydrogen atoms, but still too hot to form stars. As free charged particles became neutral atoms, the primordial cosmic fireball extinguished, and light that had been trapped by free charges could now travel through space unimpeded. This light, called the Cosmic Microwave Background radiation (CMB), is the “afterglow” of the Big Bang, which we can detect even 14 billion years later.
Analysis of the CMB very precisely determines when the universe was born, its primary composition, and how it expanded and cooled. The CMB comprises photons of many different frequencies — an energy spectrum that peaks at the corresponding temperature of the cosmos. That spectrum peaked at 3000 K when the universe was 379,000 years old, and it peaks (relative to us) at 2.725 K today, which proves the universe has expanded by a factor of 1100 from then to now. (Energy is conserved if consistently measured in the same reference frame — the 3000 K is in the frame of the gas that emitted the CMB, and the 2.725 K is in our reference frame.)
All this is known quite precisely, but how and when the first stars formed is less certain.
The epoch between the release of the CMB and the birth of the first stars is called the cosmic Dark Ages.
Gravity pulls together any clump of gas, so it seems stars would form immediately. But gravity’s inward pull is opposed by the outward push of gas pressure. If you squeeze a balloon, it gets hotter and its pressure increases as the balloon shrinks.
For gas to collapse and form a star, gravity must overcome pressure. Two effects facilitate star formation: more gravity, and less heat.
Today, the universe contains all the elements in the Periodic Table. The heavier elements have a rich variety of electron energy levels, so they very effectively cool by radiating heat. With that cooling, even a modest cloud of gas can collapse. Hence most stars that formed “recently” are relatively small, the size of our Sun or smaller.
However, those heavy elements didn’t exist at the beginning of our universe. They were created by stars and dispersed into the cosmos when those early stars died and exploded.
The gas that formed the first stars contained only the lightest elements — hydrogen and helium — neither of which radiates heat efficiently. Hence, the first stars had to be extremely massive — hundreds of times more than our Sun — to provide enough gravity. Even then, gravity overcame gas pressure only when the cosmos had substantially cooled. Such mammoth stars have very short lives, burning out in only a few million years. By comparison, our Sun can burn for 10 billion years.
That’s our current best understanding of the formation of the first stars. But until now, these ideas have lacked the life-blood of science: definitive data.
EDGES says they have detected changes in the CMB caused by the first stars. If confirmed, those changes could provide a wealth of information about this era.
Using a 6-foot antenna in the remote Outback of Australia, EDGES found a 0.1% divot in the CMB spectrum — missing radio waves near 78 MHz, as sketched below. For clarity, the dip has been greatly exaggerated and relocated in this graph of CMB intensity versus wavelength. This dip is attributed to light absorption by hydrogen atoms warmed by ultraviolet light from the first stars.
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Here’s how this works. Every element has a unique set of spectral lines — frequencies at which it absorbs and emits light. Neutral hydrogen has a very distinctive “21-cm line” at 1420 MHz. With just neutral hydrogen and the CMB, there would be no dip. Atoms would absorb and reemit CMB 1420 MHz photons — creating a dip and immediately refilling it — leaving the spectrum unchanged. Starlight is the essential third ingredient in creating a dip.
The first stars were extremely massive and extremely bright. Their ultraviolet radiation changed the state of nearby hydrogen atoms, elevating their electrons to more energetic orbits. When such excited atoms absorb 1420 MHz CMB photons — creating a dip — they return to the ground states by emitting two photons instead of one. These emitted photons have a broad energy spectrum — they add a tiny bit to the spectrum here and there, but do not refill the dip.
As the first stars exhausted their nuclear fuels, they exploded, seeding the cosmos with heavier elements, and leaving behind black holes and neutron stars. X-ray radiation from the latter gradually ionized the cosmic gas, splitting apart neutral hydrogen atoms in a process called reionization. This ended CMB absorption — no more dips.
The location and width of the dip indicate that the first stars were born 180 million years after the Big Bang, and that reionization began 250 million years after the Big Bang.
Announcements of major new scientific discoveries are carefully scrutinized. Other scientists have already written several papers supporting and critiquing EDGES. One paper noted that EDGES fitted their data with a complex function with 9 variable parameters — a huge number for this sort of analysis. A famous scientist once said: “With five parameters, I can fit an elephant” (meaning any shape at all). These critics said with fewer parameters they could fit the EDGES measurements with no dip at all.
This experiment will be repeated and reanalyzed by several independent groups. One day, these efforts will likely reach some consensus — time will tell.
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Best Regards,
Robert
June 20th, 2018
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