“I see a lot of new faces. But, you know the old saying, ‘out with the old, in with the nucleus.’”
-The Simpsons
Looking around the Universe today, there’s no doubt that there’s
plenty of hydrogen and helium around; after all, it’s the nuclear fusion
of hydrogen
into helium that powers the vast majority of stars illuminating the entire cosmos!
Image credit: ESA/Hubble, NASA and H. Ebeling.
But here on Earth, hydrogen and helium are only a small part of the
world we inhabit. By mass, hydrogen and helium combined make up far less
than 1% of the Earth, and even if we restrict ourselves to the Earth’s
crust, it’s still just a tiny percentage compared to the other, heavier
elements.
Image credit: Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS / Wikimedia user michbich.
Practically all of these heavy elements were formed in generations of
stars: stars that lived, burned their fuel into heavier elements, died
and shed their heavy, enriched elements back into the cosmos, and were
incorporated into the next generations of stars and — when the heavier
elements became abundant enough — rocky planets.
Image credit: NASA / Lynette Cook.
But the Universe didn’t start off with these heavier elements at all. In fact, if you’ll remember what the Big Bang says,
the Universe is expanding (and cooling) now, meaning that all the
matter in it was closer together — and the radiation in it was hotter —
in the past. If you go back to a sufficiently early time, you’ll find
that the density was high enough and the temperature was hot enough that
you couldn’t even form neutral atoms without them immediately being
blasted apart! When the Universe cooled through that phase, that’s when
neutral atoms formed for the first time, and where the cosmic microwave background comes from.
Image credit: Pearson / Addison Wesley, retrieved from Jill Bechtold.
At that time, the Universe was made out of about 92% hydrogen atoms
and 8% helium atoms by number (or about 75-76% hydrogen and 24-25%
helium by mass), with trace amounts of lithium and beryllium, but not
much else. But you might wonder how it got to have exactly that ratio?
After all, it didn’t have to be that way; if the Universe was hot and
dense enough to undergo nuclear fusion early on, why did it only fuse
atoms up to helium, and why didn’t
more of the Universe become helium than it did?
To find the answer, we need to go
way back in time. Not just
to the first few hundred thousand years of the Universe, when it was
making the first atoms, nor even to the first years, days, or hours. No,
we need to go back to when the temperatures were so high, when the
Universe was so hot, that not only could atomic nuclei not form (for
they’d be immediately be blasted apart), but to a time when the Universe
was so hot that the Universe was filled with nearly equal amount of
matter-and-antimatter, when it was just a fraction of a second old!
Image credit: James Schombert of the University of Oregon.
It was once so hot that the Universe was filled with
nearly
equal amount of matter and antimatter: protons and antiprotons, neutrons
and antineutrons, electrons and positrons, neutrinos and antineutrinos,
and of course photons (which are their own antiparticle), among others.
(They’re not
exactly equal; see here for more on that.) When the Universe is hot — and by hot, I mean
above
the temperature needed to spontaneously create a matter/antimatter pair
from two typical photons — you get huge amounts of that form of matter
and antimatter. They get spontaneously created from photons just as
quickly as they find one another and annihilate back into photons. But
as the Universe cools, those matter/antimatter pairs begin to annihilate
faster, and it becomes more difficult to find photons energetic enough
to make them. Eventually, it cools enough that all the exotic particles
go away, and all the antiprotons and antineutrons annihilate with
protons and neutrons, leaving only a small asymmetry of matter (in the
form of protons and neutrons) over antimatter, bathed in a sea of
radiation.
Image credit: me, background by Christoph Schaefer.
At this point, when the Universe is a fraction of a second old, there
are roughly equal amounts of protons and neutrons: about a 50/50 split.
These protons and neutrons will eventually become the atoms in our
Universe, but they’ve got a lot to go through first. On the other hand,
electrons (and positrons) are much lighter, so they still exist in huge
numbers (and at great energies) for a while longer.
Image credit: Addison-Wesley, retrieved from J. Imamura / U. of Oregon.
It’s still hot enough that protons and neutrons can convert into one
another very easily: a proton can combine with an electron to make a
neutron and (an electron) neutrino, while a neutron can combine with (an
electron) neutrino to make a proton and an electron. While there aren’t
that many protons and neutrons in the Universe at this time, electrons
and neutrinos outnumber them by around a billion-to-one. This is why,
early on, there’s about a 50/50 split of protons and neutrons.
Neutrons, as you’ll remember, are
slightly heavier than
protons: by about 0.2%. As the Universe cools (and the excess positrons
annihilate away), it becomes rarer and rarer to find a proton-electron
pair with enough energy to create a neutron, while it’s still
relatively
easy for a neutron-neutrino pair to create a proton-electron pair. This
converts a substantial fraction of neutrons into protons during the
first one-to-three seconds of the Universe. By time these interactions
have become insignificant, the proton-to-neutron ratio has changed from
about 50/50 to 85/15!
Image credit: Smith, Christel J. et al. Phys.Rev. D81 (2010) 065027.
Now, these protons and neutrons are abundant, hot, and dense enough
that they can fuse together into heavier elements, and believe me,
they’d
love to. But photons — particles of radiation – outnumber protons-and-neutrons by more than a
billion to one, so for
minutes
of the Universe expanding and cooling, it’s still energetic enough that
every time a proton and neutron fuse together to form deuterium, the
first stepping-stone in nuclear fusion, a high-enough energy photon
immediately comes along and blasts them apart! This is known as the deuterium bottleneck, as deuterium is relatively fragile, and its fragility prevents further nuclear reactions from occurring.
Image credit: me, modified from Lawrence Berkeley Labs.
In the meantime, while the minutes tick by, something else is going
on. A free proton is stable, so nothing happens to them, but a free
neutron is
unstable; it will decay into a proton, electron, and
an (electron) antineutrino with a half-life of about ten minutes. By
time the Universe has cooled enough that the created deuterium wouldn’t
be immediately be blasted back apart, more than three minutes have gone
by, further changing the 85%-proton/15%-neutron split to nearly 88%
protons and just a hair over 12% neutrons.
Image credit: Ronaldo E. de Souza.
Finally, with deuterium forming, nuclear fusion can proceed, and it
proceeds extremely rapidly! Through a couple of different fusion chains,
the Universe is still hot and dense enough that pretty much every
neutron around wind up combining with one other neutron and two protons
to form helium-4, an isotope of helium that’s much more energetically
stable than deuterium, tritium, or helium-3!
Images taken from LBL, stitched together by me.
By time this happens, though, the Universe is nearly four minutes
old, and is far too diffuse and cold to undergo the next major step of
fusion that happens in stars, which is to fuse three helium-4 atoms into
carbon-12; that process will have to wait tens of millions of years
until the Universe’s first stars form!
But these nuclei are stable, and there will also be a trace amount of
helium-3 (which tritium will also decay into, eventually), deuterium
(hydrogen-2), and very small amounts of lithium (and probably even
smaller amounts of beryllium) formed by very rare fusion reactions.
Image credit: NASA, WMAP Science Team and Gary Steigman.
But the overwhelming majority of neutrons — 99.9%+ of them — wind up
locked up in helium-4 nuclei. If the matter in the Universe contained
just a hair over 12% neutrons and just a hair under 88% protons
just prior
to nucleosynthesis (the fusion into heavier elements), that means that
all of those neutrons and and equal amount (just over 12% of the
Universe) of protons winds up becoming helium-4: a total of 24-to-25% of
the mass, leaving 75-to-76% of the Universe as protons, or hydrogen
nuclei.
Image credit: Ned Wright, via his excellent Cosmology tutorial at UCLA.
So that’s why, by mass, we say 75-76% was hydrogen and 24-25% was helium. But each helium nucleus is around
four times the mass of a hydrogen nucleus, which means that, by
number of atoms, the Universe is around 92% hydrogen and 8% helium.
This primordial, unprocessed material has actually been detected observationally, and is one of the three cornerstones of the Big Bang, along with Hubble expansion and the cosmic microwave background.
And that’s where all the elements in the Universe started from!
Everything you are, everything you know, and every material object
you’ve ever interacted with came from this primordial sea of protons and
neutrons, and was once a mere collections of hydrogen and helium atoms.
And then the Universe happened…
Image credit: NASA / JPL-Caltech / Spitzer / IRAC / N. Flagley and the MIPSGAL team.
and here it all is! And that’s where — if you go way, way back — all the atoms came from.
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