Age of the universeDiame
ter^
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f^ th
e^observableuniverse0 10 -32second 1 microsecond 0.01 second 20 minutes 380,000 years 13.8 billion yearsBig
BangProtons
formNuclear
fusion
beginsNuclear
fusion
endsModern
universeCosmic microwave backgroundNeutral
hydrogen
formsInflationWWW.ASTRONOMY.COM 63
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Unfortunately, we
cannot answer all
questions submitted.If we extrapolate even further back in time, we can
imagine a hypothetical state that may have been present
about 10–43 seconds after the Big Bang. The laws of phys-
ics as we currently understand them — including general
relativity and quantum mechanics — don’t allow us to
extrapolate any further back than this so-called Planck
time. (Read more on this cosmic era in Astronomy’s Apr i l
2022 issue.) At the Planck time, the region that is now
our observable universe would have been only a fraction
of a millimeter in diameter, or smaller than the size of a
pinhead.
You might think that at such incredible densities, the
matter and energy in our early universe would have col-
lapsed, resulting in the formation of black holes. Black
holes form, however, only when the matter and energy
is distributed unevenly. In the early universe, the energy
was distributed in an almost perfectly uniform way
throughout space. The homogeneity of the early universe
would have prevented any — or at the very least, many
— black holes from forming.
Dan Hooper
Senior Scientist, Fermi National Accelerator Laboratory,
Batavia, Illinois
QI
WHAT WILL WHITE DWARFS,
NEUTRON STARS, AND BROWNDWARFS LOOK LIKE AT THE VERY END
OF THEIR LIVES, WHEN THEY NO LONGER
EMIT RADIATION?
Bill Zuna
Tallahassee, Florida
AI
Let’s look at a white dwarf first. These stellar
remnants are the remains of Sun-like stars andare made mostly of carbon and oxygen. A white dwarf ’s
outer shell is so hot that it will radiate visible light for
about 10^10 — that’s 1 followed by 10 zeros — years. By
then, the atoms will have cooled down enough that they
crystallize into a giant diamond. However, this diamond
does not last forever. As time marches on for another
1038 years, the protons and neutrons inside all the atoms
will disintegrate and produce scant traces of light. First,
all the atoms will break down into hydrogen. Then these
hydrogen atoms will slowly disappear through a hypo-
thetical process known as proton decay.
To understand how long that would take, imagine if
you were to count every person on Earth (about 8 billion)
at a rate of one per year. By the time you finished, the
white dwarf would no longer shine. Then, if you were to
count every atom in every person on Earth — there are
about 10^28 atoms in a human body — at a rate of one atom
per year, the star will have virtually disintegrated by the
time you were done.
A neutron star is the remnant of a massive star that
has run out of fuel, exploded, and collapsed into a super-
dense star. Like a white dwarf, a neutron star will cool
over about 10^10
years to a point
where it no longer
emits visible light.
However, unlike
white dwarfs, neu-
tron stars have a
thin crust sur-
rounding densely
packed neutrons.
Over the next 10^38
years, scientists
believe the crust
will disintegrate
thanks to proton
decay. Eventually the gravitational force drops and the
star expands into something reminiscent of a white
dwarf (which now only has another 10^38 years to live).
Finally, a brown dwarf is barely visible to begin with.
It doesn’t fuse hydrogen in its core, meaning it’s not a
true star. It might fuse deuterium, or heavy hydrogen,
but that fusion will stop after a mere 10^8 years. Then it
too will slowly succumb to proton decay over 10^38 years.
It’s a very bleak future, but there is plenty of time until
then!
S.H.C. Cabot
Graduate Student, Department of Astronomy, Yale University,
New Haven, ConnecticutGROWING FROM THE BIG BANG