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It
was morning in the universe and much colder than anyone had expected
when light from the first stars began to tickle and excite their dark
surroundings nearly 14 billion years ago.
Astronomers
using a small radio telescope in Australia reported on Wednesday that
they had discerned effects of that first starlight on the universe when
it was only 180 million years old. The observations take astronomers
farther back into the mists of time than even the Hubble Space Telescope
can see and raised new questions about how well astronomers really know
the early days of the cosmos, and about the nature of the mysterious
so-called dark matter whose gravity sculpts the luminous galaxies.
“We
have seen indirectly evidence of very early stars in the universe —
stars that would have formed by the time the universe was only 180
million years old,” said Judd Bowman of Arizona State, leader of the
experiment known as EDGES, for Experiment to Detect Global EoR Signature, in an email. Dr. Bowman and his colleagues published their results in Nature Wednesday.
The
presence of stars manifested itself as a telltale dip in the intensity
of a bath of radio waves, so-called cosmic microwaves, leftover from the
fires of creation itself. The dip meant that cosmic energy was being
absorbed by primordial clouds of hydrogen gas that hung over the
universe like a fog, but whose atoms had been thrown out of balance by
the sudden presence of starlight.
The
presence of the dip, at a characteristic wavelength of hydrogen,
confirmed earlier predictions from models of how and when the stars were
born. But the depth of the dip and the amount of the absorption was a
surprise. It suggested that the gas inhabiting the cosmos was only half
as hot as astronomers had calculated — only about 3 kelvin above
absolute zero, or minus 454 Fahrenheit.
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“This
is difficult to explain based on our current knowledge and assumptions
about astrophysical processes in the early universe,” Dr. Bowman said.
One
possibility, suggested by Rennan Barkana of Tel Aviv University in
Israel, is that the primordial hydrogen could have gotten chilled by
interacting with the dark matter that also permeates the cosmos.
“If
true, this would be the first clue about the properties of dark matter,
beyond its gravitational pull which is how its presence has been
inferred,” said Dr. Barkana, who published his idea in an accompanying paper in Nature.
How
this all played out was the result of a subtle dance of atomic physics
and thermodynamics — the study of heat. In its early days before the
stars lit up, the universe was a fog of hydrogen and helium that had
been synthesized during the first three minutes of time and that was now
basking in the fading heat of the
Big Bang.
Hydrogen
in empty space is prone to radiate radio waves with a wavelength of 21
centimeters. At first the gas and the microwave were in tune with each
other, and the hydrogen was emitting just as much as it received from
the background radiation bath.
But
when the stars began to turn on, ultraviolet radiation from them altered
the energy levels of the electrons in the hydrogen atoms, knocking them
out of sync with the microwaves — “decoupling” them in the physics
vernacular. Since the gas was already physically much colder than the
radiation, it began to absorb the 21-centimeter waves from the cosmic
background, creating a deficit, or a dip.
The shock was how great a dip that was and thus how much colder the hydrogen was than cosmologists had figured.
Enter cold dark matter.
“The
only known cosmic constituent that can be colder than the early cosmic
gas is dark matter,” Dr. Barkana wrote in his Nature paper.
Astronomers
know that dark matter makes up about a quarter of the universe by
weight — way more than atomic matter — from its gravitational effects on
stars and galaxies. The leading explanation has been that it consists
of clouds of subatomic particles left over from the Big Bang. They’re
called wimps, for weakly interacting massive particles, and are hundreds
of times as massive as a hydrogen atom. Because these particles are so
massive they are also slow, or “cold” in cosmic jargon.
In
theory, they should be passing through our bodies and everything else
by the millions every second. But over the last three decades increasingly sensitive attempts to detect these particles directly have failed, and theorists are beginning to consider other more complicated models of what they call “the dark sector.”
Now
the EDGES observations might have opened a new window into that dark
realm. And any progress in identifying dark matter could revolutionize
particle physics.
The
idea that dark matter could have cooled the primordial hydrogen would
imply that dark matter particles are only a few times heavier than
hydrogen atoms, “well below the commonly predicted mass of weakly
interacting massive particles,” Dr. Barkana explained in his Nature
paper.
It would mean that radio astronomers have a way of getting a grip on dark matter.
None
of this is for certain. Yet. Both Dr. Bowman and Dr. Barkana emphasized
that the observations need to be confirmed by other instruments and
experiments. The EDGES result was based on averaging observations over
the whole sky. But new projects in the works, like the Square Kilometer Array in Australia and South Africa
will be able to measure these temperature discrepancies in different
parts of the sky and track the different evolution of dark and luminous
matter.
A version of this article appears in print on , on Page D3 of the New York edition with the headline: Glimpsing a Cosmic Dawn. Order Reprints | Today’s Paper | Subscribe
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