- 4458 - UNIVERSE - and Cosmic Inflation? - Our Universe arose in the aftermath of “cosmic inflation”, triggering the hot Big Bang some 13.8 billion years ago that eventually gave rise to us. The Universe passed through many epochs, from free quarks and gluons to stable protons and neutrons to neutral atoms to stars, galaxies, planets, and more.
----------------------------------- 4458 - UNIVERSE - and Cosmic Inflation?
- We can pinpoint the precise time at which
these various epochs occurred, including the dark energy-dominated era. Today, it’s now 13.8 billion years since
the Big Bang occurred. Our observable Universe extends for 46.1 billion
light-years in all directions, and is made of:
-
----------------------- 68% dark energy,
----------------------- 27% dark matter,
----------------------- 4.9% normal (atom-based) matter,
---------------------- 0.09% neutrinos,
----------------------- 0.01% radiation,
–
with no hint of
other components like spatial curvature, cosmic strings, domain walls, or any
other weird stuff we can imagine.
-
- If we were to run the clock backward,
however, we’d find that dark energy wasn’t always dominant. There was a time
when “matter dominated”, and before that, when radiation did. There was a time
when there were no stars, no neutral atoms, no atomic nuclei, no protons and
neutrons, and even no massive particles.
No us.
-
- But how do we know precisely when these
events and epochs occurred? The epochs
of the universe are tagged with a specific time. The “Hadron epoch”, for
example, started at 10^-6 seconds.
-
- When dark energy reaches a number near
100% in the future, the energy density of the Universe (and, therefore, the
expansion rate) will remain constant arbitrarily far ahead in time. Owing to
dark energy, distant galaxies are already speeding up in their apparent
recession speed from us.
-
- In the beginning as far back as we can
trace things the Universe was inflating. That means it was expanding
relentlessly, doubling in scale in all directions with each tiny
fraction-of-a-second (something like 10^-35 seconds) that goes by. After only a
few hundred of those doublings, a tiny quantum fluctuation occurring on Planck
scales, the smallest scale we can describe before the known laws of physics
break down, gets stretched to scales even larger than the observable Universe.
-
- Because of this rapid expansion, the
Universe quickly empties; the only thing in it is empty space, and the large
amount of energy tied up in whatever quantum field caused inflation, plus the
small amount of “fluctuation energy” arising from the stretched quantum
fluctuations occurring on all scales.
-
- Then inflation ends, and that field energy
gets converted into all the quanta we know of. Particles and antiparticles of
all types, including photons, get spontaneously created at very high energies
and very large densities. They’re almost uniformly distributed, with an average
“overdense” region and an average “underdense” region departing from the
average density by just one part in 30,000. From this point on, the Universe
always expands, cools, and gravitates, and all sorts of events in the evolutionary
history of our Universe occur.
-
- As the Universe expands, the matter density
dilutes, but the radiation also becomes cooler as its wavelengths get stretched
to longer, less energetic states. Dark energy’s density, on the other hand,
will remain constant if it behaves as is currently thought: as a form of energy
intrinsic to space itself. These three components, together, dictate how the
Universe expands at all times from the Big Bang until the present day and
beyond.
-
- At the start, these quanta all collide with
one another at tremendous rates: quadrillions of times per second. They’re all
massless, so they’re all moving at the speed of light, possessing very large
amounts of energy. But as the Universe expands, it also cools: remember that
all particles, massive or massless, can be described by a wave as well, and the
wavelength of any wave determines its energy. As the Universe expands, the
wavelength of all waves get stretched, meaning they lose energy and the temperature
of the Universe drops.
-
- At some point, the Universe cools enough so
that the shortest-lived, most unstable particles and antiparticles begin
decaying. The Higgs and electroweak
symmetries break, giving rise to rest mass and separating the weak and
electromagnetic forces. The
quarks-and-antiquarks and gluons, previously free particles, become grouped
into protons, neutrons, and other bound states known as hadrons. The antimatter annihilates away with most of
the matter, creating a great bath of radiation and just a small population of
excess matter
-
- Nuclear fusion can occur without composite
nuclei immediately being blasted apart,
matter overtakes radiation as
the dominant component of the Universe, neutral atoms can stably form, giving
rise to a Universe that’s now transparent to visible light.
-
- The first stars form, paving the way for
the era of stars and galaxies, and then
dark energy overtakes matter as the dominant component of the Universe,
ensuring our cosmic fate to be driven away from all unbound galaxies and
groups/clusters of galaxies.
-
- That’s a very rough outline of the history
of the Universe.
-
- The Solar System gives way to the Milky
Way, which gives way to nearby galaxies which then give way to the large-scale
structure and the hot, dense plasma of the Big Bang at the outskirts. If we
were to shrink the Sun down to a grain of sand, then the observable Universe
would only be a little over a hundred billion kilometers in radius, containing
around 2 sextillion “grains of sand” (stars) strewn across that enormous
volume.
-
- Now for the big question: how can we
determine when all of these things happen?
-
- The way we normally do this is
straightforward, even if the mathematics itself is a little difficult. Where we
start is by recognizing the following three things. The Universe presently has a “bath” of
background radiation that’s left over from the Big Bang, with an average
temperature, today, of 2.7255 K.
-
- The Universe presently has a specific
“size” or “scale” to it, which we believe is very close to 46.1 billion
light-years in radius, but which we can just call “the size today.” And that, because the Universe is
continuously expanding and cooling, it was smaller and hotter and denser in the
past, and we can determine “How hot was it?” at any point in time simply by
recognizing that if you take the temperature today and divide it by the ratio
of “the scale of the Universe back then” to “the scale of the Universe today,”
you’ll get the temperature of the Universe back at any epoch you desire.
-
- Therefore, if you know either: what was the temperature/energy at which
this specific event-or-transition occurred,
or what was the scale, relative to today’s scale, at which this specific
event-or-transition occurred, you can do
the math to figure out precisely when, in terms of our cosmic history, these
events and eras took place.
-
- A visual history of the expanding Universe
includes the hot, dense state known as the Big Bang and the growth and
formation of structure subsequently. The full suite of data, including the
observations of the light elements and the cosmic microwave background, leaves
only the Big Bang as a valid explanation for all we see.
-
- As the Universe expands, it also cools,
enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and
finally galaxies to form. Without the Higgs giving mass to the particles in the
Universe at a very early, hot stage, none of this would have been possible.
-
- Figuring out either temperature/energy or
relative scale at which certain events occurred is relatively straightforward.
For events that occur in the context of particle/high-energy physics, we simply
need to look to our experimental data to figure out at what energy/temperature
they occur at. For events that occur throughout the history of the Universe,
these can either be measured or calculated using one of two straightforward
methods.
-
- For events that can be determined
observationally, we can measure the redshift at which they occur/exist, by
comparing the observed emission/absorption lines generated by atomic or
molecular transitions with the same lines generated by that same molecule or
atom within the lab. The ratio of observed-to-rest-frame wavelength equals the
ratio of “the scale of the Universe back then” to “the scale of the Universe
today,” and that ratio, minus the number 1, is the definition of redshift.
-
- For events that can occurred at some point
in the Universe’s past, we can calculate “What was the scale of the Universe
back then?” simply by numerically integrating the equation that governs how the
Universe expands over time: the first “Friedmann equation”.
-
- The first Friedmann equation details the
Hubble expansion rate squared on the left hand side, which governs the
evolution of spacetime. The right side includes all the different forms of
matter and energy, along with spatial curvature (in the final term), which
determines how the Universe evolves in the future. This has been called the
most important equation in all of cosmology and was derived by Friedmann in
essentially its modern form back in 1922.
-
- If you can figure out what the Universe’s
scale factor was when a specific transition occurred, then you can do a little
math to determine “At what time in the Universe’s history was the Universe this
specific size/scale?” This requires
some numerical integration, but there’s a short-cut you can use that works
pretty well for the first few billion years of our cosmic history: assuming the
Universe was either made 100% out of radiation (which works for the first
~10,000 years), or assuming that the Universe was made 100% out of matter
(which works for the next ~7 billion years).
-
- The short-cut is as follows. If your Universe is made 100% of radiation,
then it expands according to a simple rule: the scale factor grows with time as
a ~ t^½. If your Universe is made 100%
of matter, then it expands according to a simple rule: the scale factor grows
with time as a ~ t^⅔. And if your
Universe is made 100% of dark energy, which dominates the expansion at late
(modern) times, the scale factor grows exponentially: as a ~ eHt.
-
- If we fill in the in-between stages, or
just calculate the whole thing directly without using short-cuts, we can
determine the scale/size of the Universe as a function of cosmic time.
-
- That’s it, as long as you realize that the ratio of the scale of the Universe
at any time to the scale of the Universe today is equal to the temperature of
the Universe at any time to the temperature of the Universe today, you can calculate at what time any event
occurs if you know either how big the Universe was at that time relative to
today or you know what the temperature of the Universe was at that time
relative to today.
-
- All we need to remember is that the
temperature of the Universe, today, is 2.725 K, and that the
size/scale/redshift of the Universe today is 46.1 billion light-years/defined
as 1/defined as 0, and also that the age of the Universe, today, is 13.8
billion years after the Big Bang.
-
- As long as you start with today’s
composition of the Universe — 68% dark energy, 31.9% matter, 0.09% neutrinos
and 0.01% photons — and you recognize that everything behaves as radiation when
it moves at or very close to the speed of light, you can’t go wrong.
-
- The quantum fluctuations inherent to
space, stretched across the Universe during cosmic inflation, gave rise to the
density fluctuations imprinted in the cosmic microwave background, which in
turn gave rise to the stars, galaxies, and other large-scale structures in the
Universe today.
-
- Inflationary era: This is a tricky one,
but only in the sense that we don’t know when, how, or even if it had a
beginning, but we do know that it lasted at least some ~10^-33 seconds, and
that when it ended, the hot Big Bang began.
-
- Free, unbound, massless
particle-and-antiparticle era: This is what you might think of as the
Universe’s “primordial soup,” where every possible collision that you can
imagine occurs in great abundance. There are no bound structures; there are no
stable configurations; any particle that you can track likely gets annihilated
and transforms into other particles many, many times over. This lasts from the
end of inflation right up until the Universe is about ~10^-10 seconds old, or
~100 picoseconds.
-
- Massive particle-and-antiparticle/quark
gluon plasma era: After about the first ~10^-10 seconds, the Higgs and
electroweak symmetries break, separating the electroweak force into the
electromagnetic and weak forces, and giving mass to the Universe.
-
- This transition is also the final
opportunity for the Universe to create a matter-antimatter asymmetry; if it
hasn’t happened before now, this is its last opportunity. As it expands and
cools, the heavier quarks-and-antiquarks decay away, as do the tau-antitau
lepton pairs. This continues until the Universe is about one microsecond
(~10^-6 seconds) old, when the next major transition happens.
-
- In the very early Universe, there were
tremendous numbers of quarks, leptons, antiquarks, and antileptons of all
species. After only a tiny fraction-of-a-second has elapsed since the hot Big
Bang, most of these matter-antimatter pairs annihilate away, leaving a very
tiny excess of matter over antimatter. How that excess came about is a puzzle
known as baryogenesis, and is one of the greatest unsolved problems in modern
physics.
-
- Hadron era: Prior to this point,
quarks-and-antiquarks and gluons were still in a primordial soup: the
quarks-and-antiquarks had masses, but wouldn’t form bound states, as the
energies and densities were too great. But now, groups of three quarks form
baryons like protons and neutrons, groups of three antiquarks form
anti-baryons, and quark-antiquark pairs form mesons. -
- All of the mesons are unstable, and decay
quite quickly, while the remaining anti-baryons all get annihilated away by
baryons, producing an enormous bath of radiation. At the end, there are ~1 billion photons left over for every
baryon, but no surviving antibaryons. This lasts until the Universe is about 1
second old, and a handy rule-of-thumb is that an age of ~1 second corresponds
to an average energy-per-particle of 1 MeV, or a temperature of ~10^10 K: ten
billion degrees.
-
- Nuclear era: At an age of ~1 second,
neutrinos stop interacting regularly with the remaining particles and
antiparticles in the Universe, and almost immediately after, the remaining
positrons annihilate with a huge excess of electrons, producing even greater
numbers of photons, and heating them so they’re slightly hotter by about 40% than
the neutrinos.
-
- Nuclear fusion is attempting to occur
between protons and neutrons, but the photons blast them apart until the
Universe reaches about ~3 minutes of age. At last, now that it’s cool enough,
nuclear fusion occurs, forming deuterium, tritium, helium-3, helium-4, and both
lithium-7 and beryllium-7. By the time the Universe is about 4-to-20 minutes
old, all of this is done.
-
- A Universe where electrons and protons are
free and collide with photons, transitions to a neutral one that’s transparent
to photons as the Universe expands and cools.
It’s the spectacular two-photon transition in a hydrogen atom which
enables the Universe to become neutral exactly as we observe it.
-
- Plasma era: It’s now too cold and too
sparse for fusion reactions to occur, and all of the tritium decays to helium-3
while all of the beryllium-7 decays to lithium-7. Protons and the other atomic
nuclei would love to bind together to electrons, but they can’t without getting
immediately blasted apart by a high-energy photon.
-
- At an age of about ~9,000 years, radiation
quits being the dominant component of the Universe, replaced by the combination
of normal-and-dark matter. This lasts until the Universe reaches an age of
about 380,000 years, and a temperature of merely ~3,000 K.
-
- Atomic era: At last, at this point, 380,000
years after the Big Bang, the Universe forms neutral atoms, and is now
transparent to light, including the radiation left over from the Big Bang. But
because the densest overdense regions and the least-dense underdense regions
are still so close to the cosmic average, it takes time for gravitation to
collapse these atoms down to where they can form stars. Although the exact
number isn’t yet known, we can ballpark it to last until the first stars form,
100 million years after the Big Bang.
-
- Stellar-and-galactic era: Beginning roughly
100 million years after the Big Bang, now “Let there be light” has officially
occurred for the second time: with the birth of stars and star clusters. These
will grow and merge into galaxies, galaxy groups, and clusters, and will align
along great cosmic walls, eventually forming the modern cosmic web.
-
- Over time, gravitational interactions will
turn a mostly uniform, equal-density Universe into one with large
concentrations of matter and huge voids separating them. For as long as
radiation is still important, exerting an outward pressure even once the
Universe becomes matter-dominated, the growth of matter imperfections is very
small.
-
- Although this process persists far into the
future, we’ve already entered the final one: the one that describes our
Universe now and will describe our Universe for all-time afterward. The dark energy era: There are two ways to
define its onset, depending on whether you define “dark energy dominates the
Universe” as talking about when the recession speed of a distant galaxy stops
slowing down and starts speeding up, or when dark energy becomes the dominant
form of energy in the Universe, overtaking matter.
-
- By the first definition, dark energy
dominates the expansion rate of the Universe at an age of 7.8 billion years
after the Big Bang. By the second definition, that domination is delayed until
the Universe is 10.4 billion years after the Big Bang. Right at the same time
that the first photosynthetic organisms evolve on Earth, dark energy passes
both dark matter and normal matter combined to dominate the energy content of
the Universe.
-
- And that’s the way it will be, forever and
ever into the future, at least to the best of our knowledge. This timeline can
be made more-or-less granular, but that’s how we know it and how we figure it
out! Enough said.
-
-
May 5, 2024 UNIVERSE
- and Cosmic Inflation? 4458
------------------------------------------------------------------------------------------
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--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
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--- to:
------
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------ “Jim Detrick” -----------
--------------------- --- Monday, May 6, 2024
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