- 3904 - BIG BANG THEORY - the best math can give us? We now know the big bang theory is (probably) not how the universe began. Where did all this come from? In every direction we care to observe, we find stars, galaxies, clouds of gas and dust, tenuous plasmas, and radiation spanning the gamut of wavelengths: from radio to infrared to visible light to gamma rays.
------------ 3904
- BIG BANG
THEORY - the best math can give us?
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- No matter
where or how we look at the universe, it’s full of matter and energy absolutely
everywhere and at all times. And yet, it’s only natural to assume that it all
came from somewhere.
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- The
universe as we see it is expanding, rarifying (getting less dense), and cooling.
Although it’s tempting to simply extrapolate forward in time, when things will
be even larger, less dense, and cooler, the laws of physics allow us to
extrapolate backward just as easily.
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- Long ago,
the universe was smaller, denser, and hotter. How far back can we take this
extrapolation? Mathematically, it’s tempting to go as far as possible: all the
way back to infinitesimal sizes and infinite densities and temperatures, or
what we know as a “singularity”. This idea, of a singular beginning to space,
time, and the universe, was long known as the Big Bang.
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- The Big
Bang has its roots in both theoretical and experimental / observational realms.
On the theory side, Einstein put forth his general theory of relativity in
1915: a novel theory of gravity that sought to overthrow Newton’s theory of
universal gravitation.
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- In 1916,
Karl Schwarzschild found the solution for a pointlike mass, which describes a
nonrotating black hole.
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- In 1917,
Willem de Sitter found the solution for an empty universe with a cosmological
constant, which describes an exponentially expanding universe.
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- From 1916
to 1921, the Reissner-Nordström solution, found independently by four
researchers, described the spacetime for a charged, spherically symmetric mass.
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- In 1921,
Edward Kasner found a solution that described a matter-and-radiation-free
universe that’s anisotropic: different in different directions.
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- In 1922,
Alexander Friedmann discovered the solution for an isotropic (same in all
directions) and homogeneous (same at all locations) universe, where any and all
types of energy, including matter and radiation, were present.
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- Friedmann's
theory appeared to describe our universe on the largest scales, where things appear
similar, on average, everywhere and in all directions. And, if you solved the governing equations for
this solution, the Friedmann equations, you’d find that the universe it
describes cannot be static, but must either expand or contract.
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- In the
1910s, astronomer Vesto Slipher started observing certain nebulae, which might
be galaxies outside of our Milky Way, and found that they were moving fast: far
faster than any other objects within our galaxy. The majority of them were moving away from us,
with fainter, smaller nebulae generally appearing to move faster.
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- Then, in the
1920s, Edwin Hubble began measuring individual stars in these nebulae and
eventually determined the distances to them. Not only were they much farther
away than anything else in the galaxy, but the ones at the greater distances
were moving away faster than the closer ones.
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- Georges
Lemaître was the first, in 1927, to recognize this. Upon discovering the
expansion, he extrapolated backward, theorizing that you could go as far back
as you wanted: to what he called the “primeval atom”. In the beginning, he
realized, the universe was a hot, dense, and rapidly expanding collection of
matter and radiation, and everything around us emerged from this primordial
state.
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- The
universe, as we see it today, is more evolved than it was in the past. The
farther back we look in space, the farther back we’re also looking in time. So,
the objects we see back then should be younger, less gravitationally clumpy,
less massive, with fewer heavy elements, and with less-evolved structure. There
should even be a point beyond which no stars or galaxies were present.
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- At some
point, the radiation was so hot that neutral atoms couldn’t stably form,
because radiation would reliably kick any electrons off of the nuclei they were
attempting to bind to, and so there should be a leftover, now cold and sparse,
bath of cosmic radiation from this time.
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- At some
extremely early time it would have been so hot that even atomic nuclei would be
blasted apart, implying there was an early, pre-stellar phase where nuclear
fusion would have occurred: “Big Bang nucleosynthesis'.
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- From that,
we expect there to have been at least a population of light elements and their
isotopes spread throughout the universe before any stars formed. With the expanding univere becomes the
cornerstone of the Big Bang:
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------------------
The growth and evolution of the large-scale structure of the universe,
of individual galaxies, and of the stellar populations found within those
galaxies all validates the Big Bang’s predictions.
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-----------------
The discovery of a bath of radiation just 3 Kelvin above absolute zero
was the key evidence that validated the Big Bang and eliminated many of its
most popular alternatives.
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The discovery and measurement of the light elements and their ratios,
including hydrogen, deuterium, helium-3, helium-4, and lithium-7, revealed not
only which type of nuclear fusion occurred prior to the formation of stars, but
also the total amount of normal matter that exists in the universe.
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---------------------
Extrapolating back to as far as your evidence can take you is a
tremendous success for science. The
earliest observable imprint is the “cosmic neutrino background”, whose effects
show up in both the microwave background (the Big Bang’s leftover radiation)
and the universe’s large-scale structure. This neutrino background comes to us,
remarkably, from just 1 second into the hot Big Bang.
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- We can trace
the hot Big Bang back some 13.8 billion years, all the way to when the universe
was less than 1 second old. Beginning
at a singularity at arbitrarily high temperatures, arbitrarily high densities,
and arbitrarily small volumes will have consequences for our universe that
aren’t necessarily supported by observations.
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- If the
universe began from a singularity, then it must have sprung into existence with
exactly the right balance of “stuff” in it, matter and energy combined, to
precisely balance the expansion rate.
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- If there
were just a tiny bit more matter, the initially expanding universe would have
already recollapsed by now. And if there were a tiny bit less, things would
have expanded so quickly that the universe would be much larger than it is
today.
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- What we’re
observing is that the universe’s initial expansion rate and the total amount of
matter and energy within it balance as perfectly as we can measure. Why?
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- If the Big
Bang began from a singularity, we have no explanation; we simply have to assert
“the universe was born this way,” or, as “initial conditions.”
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- Similarly, a
universe that reached arbitrarily high temperatures would be expected to
possess leftover high-energy relics, like magnetic monopoles, but we don’t
observe any.
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- The
universe would also be expected to be different temperatures in regions that
are causally disconnected from one another (
in opposite directions in space at our observational limits) and yet the
universe is observed to have equal temperatures everywhere to 99.99%+
precision.
-
- What cosmic
inflation gives us is an extrapolate the hot Big Bang back to a very early,
very hot, very dense, very uniform state. Inflation accomplishes this by
postulating a period, prior to the hot Big Bang, where the universe was
dominated by a large cosmological constant, the same solution found by de
Sitter way back in 1917.
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- This phase
stretches the universe flat, gives it the same properties everywhere, gets rid
of any pre-existing high-energy relics, and prevents us from generating new
ones by capping the maximum temperature reached after inflation ends and the
hot Big Bang ensues.
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- By assuming
there were quantum fluctuations generated and stretched across the universe
during inflation, it makes new predictions for what types of imperfections the
universe would begin with. Since it was
hypothesized back in the 1980s, inflation has been tested in a variety of ways
against the alternative: a universe that began from a singularity.
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- Before the
hot Big Bang, the early universe underwent a phase of exponential growth, where
any preexisting components to the universe were literally “inflated away.” When
inflation ended, the universe reheated to a high, but not arbitrarily high,
temperature, giving us the hot, dense, and expanding universe that grew into
what we inhabit today.
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- By the very
nature of inflation, it wipes out any information that came before the final
few moments: where it ended and gave rise to our hot Big Bang. Inflation could
have gone on for an eternity, it could have been preceded by some other
nonsingular phase, or it could have been preceded by a phase that did emerge
from a singularity.
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- Until the
day comes where we discover how to extract more information from the universe
than presently seems possible, we have no choice but to face our ignorance. The
Big Bang still happened a very long time ago, but it wasn’t the “beginning” we
once supposed it to be.
-
March 5, 2023 BIG
BANG THEORY - the
best math can give us? 3904
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---------------------
--- Monday, March 6, 2023
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