The Big Bang was hot, dense, uniform, and filled with matter and energy. Before that? There was nothing. Here's how that's possible.
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The Universe is an amazing place, and the way it came to be
today is something very much worth being thankful for. Although our most
spectacular pictures of space are rich in galaxies, the majority of the volume
of the Universe is devoid of matter, galaxies, and light entirely. We can only
imagine a Universe where space is truly empty. (Credit: NASA/ESA/Hubble
Heritage Team (STScI/AURA); J. Blakeslee) |
The more curious we get about the great cosmic unknowns, the
more unanswered questions our investigations of the Universe will reveal.
Inquiring about the nature of anything — where it is, where it came from, and
how it came to be — will inevitably lead you to the same great mysteries: about
the ultimate nature and origin of the Universe and everything in it. Yet, no
matter how far back we go, those same lingering questions always seem to
remain: at some point, the entities that are our “starting point” didn’t
necessarily exist, so how did they come to be? Eventually, you wind up at the
ultimate question: how did something arise from nothing? As many recent
questioners, including Luke Martin, Buzz Morse, Russell Blalack, John Heiss and
many others have written:
“Okay, you surely receive this question endlessly, but I
shall ask nonetheless: How did something (the universe/big bang) come from
nothing?”
This is maybe one of the biggest questions of all, because
it’s basically asking not only where did everything come from, but how did all
of it arise in the first place. Here’s as far as science has gotten us, at
least, so far.
A detailed look at the Universe reveals that it’s made of
matter and not antimatter, that dark matter and dark energy are required, and
that we don’t know the origin of any of these mysteries. However, the
fluctuations in the CMB, the formation and correlations between large-scale
structure, and modern observations of gravitational lensing all point towards
the same picture. (Credit: Chris Blake and Sam Moorfield) |
Today, when we look out at the Universe, the full suite of
observations we’ve collected, even with the known uncertainties taken into
account, all point towards a remarkably consistent picture. Our Universe is
made of matter (rather than antimatter), obeys the same laws of physics
everywhere and at all times, and began — at least, as we know it — with a hot
Big Bang some 13.8 billion years ago. It’s governed by General Relativity, it’s
expanding and cooling and gravitating, and it’s dominated by dark energy (68%)
and dark matter (27%), with normal matter, neutrinos, and radiation making up
the rest.
Today, of course, it’s full of galaxies, stars, planets,
heavy elements, and in at least one location, intelligent and technologically
advanced life. These structures weren’t always there, but rather arose as a
result of cosmic evolution. In a remarkable scientific leap, 20th century
scientists were able to reconstruct the timeline for how our Universe went from
a mostly uniform Universe, devoid of complex structure and consisting
exclusively of hydrogen and helium, to the structure-rich Universe we observe
today.
This snippet from a structure-formation simulation, with the
expansion of the Universe scaled out, represents billions of years of
gravitational growth in a dark matter-rich Universe. Note that filaments and
rich clusters, which form at the intersection of filaments, arise primarily due
to dark matter; normal matter plays only a minor role. (Credit: Ralf Kaehler
and Tom Abel (KIPAC)/Oliver Hahn) |
If we start from today, we can step backwards in time, and ask where any individual structure or component of that structure came from. For each answer we get, we can then ask, “ok, but where did that come from and how did that arise,” going back until we’re forced to answer, “we don’t know, at least not yet.” Then, at last, we can contemplate what we have, and ask, “how did that arise, and is there a way that it could have arisen from nothing?”
So, let’s get started.
The life we have today comes from complex molecules, which
must have arisen from the atoms of the periodic table: the raw ingredients that
make up all the normal matter we have in the Universe today. The Universe
wasn’t born with these atoms; instead, they required multiple generations of
stars living-and-dying, with the products of their nuclear reactions recycled
into future generations of stars. Without this, planets and complex chemistry
would be an impossibility.
Supernova remnants (L) and planetary nebulae (R) are both
ways for stars to recycle their burned, heavy elements back into the
interstellar medium and the next generation of stars and planets. These
processes are two ways that the heavy elements necessary for chemical-based
life to arise are generated, and it’s difficult (but not impossible) to imagine
a Universe without them still giving rise to intelligent observers. (Credits:
ESO/VLT/FORS Instrument & Team (L); NASA/ESA/C.R. O’Dell (Vanderbilt) and
D. Thompson (LBT) (R)) |
In order to form modern stars and galaxies, we need:
- gravitation to pull small galaxies and star clusters into one another, creating large galaxies and triggering new waves of star formation,
- which required pre-existing collections of mass, created from gravitational growth,
- which require dark matter haloes to form early on, preventing star forming episodes from ejecting that matter back into the intergalactic medium,
- which require the right balance of normal matter, dark matter, and radiation to give rise to the cosmic microwave background, the light elements formed in the hot Big Bang, and the abundances/patterns we see in them,
- which required initial seed fluctuations — density imperfections — to gravitationally grow into these structures,
- which require some way of creating these imperfections, along with some way of creating dark matter and creating the initial amounts of normal matter.
These are three key ingredients that are required, in the
early stages of the hot Big Bang, to give rise to the Universe as we observe it
today. Assuming that we also require the laws of physics and spacetime itself
to exist — along with matter/energy itself — we probably want to include those
as the necessary ingredients that must somehow arise.
An equally-symmetric collection of matter and antimatter (of
X and Y, and anti-X and anti-Y) bosons could, with the right GUT properties,
give rise to the matter/antimatter asymmetry we find in our Universe today.
However, we assume that there is a physical, rather than a divine, explanation
for the matter-antimatter asymmetry we observe today, but we do not yet know
for certain. (Credit: E. Siegel/Beyond the Galaxy) |
So, in short, when we ask whether we can get a Universe from
nothing or not, these are the novel, hitherto unexplained entities that we need
to somehow arise.
To get more matter than antimatter, we have to extrapolate back into the very early Universe, to a time when our physics is very much uncertain. The laws of physics as we know them are in some sense symmetric between matter and antimatter: every reaction we’ve ever created or observed can only create-or-destroy matter and antimatter in equal amounts. But the Universe we had, despite beginning in an incredibly hot and dense state where matter and antimatter could both be created in abundant, copious amounts, must have had some way to create a matter/antimatter asymmetry where none existed initially. There are many ways to accomplish this. Although we don’t know which scenario actually took place in our young Universe, all ways of doing so involve the following three elements:
- an out-of-equilibrium set of conditions, which naturally arise in an expanding, cooling Universe,
- a way to generate baryon-number-violating interactions, which the Standard Model allows through sphaleron interactions (and beyond-the-Standard-Model scenarios allow in additional ways),
- and a way to generate enough C and CP violation to create a matter/antimattery asymmetry in great enough amounts.
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