- 4601 - BIG BANG THEORY - what started it all? - The Big Bang theory describes how the universe was born in a cataclysmic explosion almost 14 billion years ago. In a tiny fraction of a second, the observable universe grew by the equivalent of a bacterium expanding to the size of the Milky Way. This early universe was extraordinarily hot and extremely dense.
-------------------------------------- 4601 - BIG BANG THEORY - what started it all?
- In 1929, the American astronomer Edwin
Hubble discovered that distant galaxies are moving away from each other,
leading to the realization that the universe is expanding. If we were to wind
the clock back to the birth of the cosmos, the expansion would reverse and the
galaxies would fall on top of each other 14 billion years ago. This age agrees
nicely with the ages of the oldest astronomical objects we observe.
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- In 1964, Arno Penzias and Robert Wilson
detected a particular type of radiation that fills all of space. This became
known as the “cosmic microwave background” (CMB) radiation. It is a kind of
afterglow of the Big Bang explosion, released when the cosmos was a mere
380,000 years old.
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- The CMB provides a window into the hot,
dense conditions at the beginning of the universe. Penzias and Wilson were
awarded the 1978 Nobel Prize in Physics for their discovery. More recently, experiments at particle
accelerators like the Large Hadron Collider (LHC) have shed light on conditions
even closer to the time of the Big Bang.
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- Our understanding of physics at these high
energies suggests that, in the very first moments after the Big Bang, the four
fundamental forces of physics that exist today were initially combined in a
single force.
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- The present day four forces are gravity,
electromagnetism, the strong nuclear force and the weak nuclear force. As the
universe expanded and cooled down, a series of dramatic changes, called phase
transitions (like the boiling or freezing of water), separated these forces.
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- Experiments at particle accelerators
suggest that a few billionths of a second after the Big Bang, the latest of
these phase transitions took place. This was the breakdown of electroweak
unification, when electromagnetism and the weak nuclear force ceased to be
combined. This is when all the matter in the universe assumed its mass.
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- Moving on further in time, the universe is
filled with a strange substance called “quark-gluon plasma”. This "primordial soup" was made up
of quarks and gluons. These are sub-atomic particles that are responsible for
the strong nuclear force. Quark-gluon plasma was artificially generated in 2010
at the Brookhaven National Laboratory and in 2015 at the LHC.
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- Quarks and gluons have a strong attraction
for one another and today are bound together as protons and neutrons, which in
turn are the building blocks of atoms. However, in the hot and dense conditions
of the early universe, they existed independently.
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- The quark-gluon plasma didn't last long.
Just a few millionths of a second after the Big Bang, as the universe expanded
and cooled, quarks and gluons clumped together as protons and neutrons, the
situation that persists today. This event is called “quark confinement”.
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- As the universe expanded and cooled still
further, there were fewer high energy photons (particles of light) in the
universe than there had previously been. This is a trigger for the process
called “Big Bang nucleosynthesis” (BBN). This is when the first atomic nuclei,
the dense lumps of matter made of protons and neutrons and found at the centers
of atoms, formed through nuclear fusion reactions, like those that power the
sun.
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- Back when there were more high energy
photons in the universe, any atomic nuclei that formed would have been quickly
destroyed by them (a process called photodisintegration). BBN ceased just a few
minutes after the Big Bang, but its consequences are observable today.
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- Observations by astronomers have provided
us with evidence for the primordial abundances of elements produced in these
fusion reactions. The results closely agree with the theory of BBN. If we
continued on, over nearly 14 billion years of time, we would reach the
situation that exists today.
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- Scientists have no direct evidence for what
came before the breakdown of electroweak unification (when electromagnetism and
the weak nuclear force ceased to be combined). At such high energies and early
times, we can only stare at the mystery of the Big Bang.
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- When we go backwards in time through the
history of the universe, the distances and volumes shrink, while the average
energy density grows. At the Big Bang, distances and volumes drop to zero, all
parts of the universe fall on top of each other and the energy density of the
universe becomes infinite. Our mathematical equations, which describe the
evolution of space and the expansion of the cosmos, become infested by zeros
and infinities and stop making sense.
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- We call this a “singularity”. Albert
Einstein's theory of general relativity describes how spacetime is shaped.
“Spacetime” is a way of describing the three-dimensional geometry of the
universe, blended with time. A curvature in spacetime gives rise to gravity.
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- But mathematics suggests there are places in
the universe where the curvature of spacetime becomes unlimited. These
locations are “singularities”. One such example can be found at the center of a
black hole. At these places, the theory of general relativity breaks down.
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- From 1965 to 1966, the British theoretical
physicists Stephen Hawking and Roger Penrose presented a number of mathematical
theorems demonstrating that the spacetime of an expanding universe must end at
a singularity in the past: the Big Bang singularity.
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- Space and time appear at the Big Bang singularity,
so questions of what happens "before" the Big Bang are not well
defined. As far as science can tell, there is no before; the Big Bang is the
beginning of “time”.
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- However, nature is not accurately described
by general relativity alone, even though the latter has been around for more
than 100 years and has not been disproven. “General relativity” cannot describe
atoms, nuclear fusion or radioactivity. These phenomena are instead addressed
by “quantum theory”.
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- Theories from "classical"
physics, such as relativity, are deterministic. This means that certain initial
conditions have a definite outcome and are therefore absolutely predictive.
Quantum theory, on the other hand, is probabilistic. This means that certain
initial conditions in the universe can have multiple outcomes.
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- “Quantum theory” is somewhat predictive,
but in a probabilistic way. Outcomes are assigned a probability of existing. If
the mathematical distribution of probabilities is sharply peaked at a certain
outcome, then the situation is well described by a "classical" theory
such as general relativity. But not all systems are like this. In some systems,
for example atoms, the probability distribution is spread out and a classical
description does not apply.
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- In the vast majority of cases, gravity is
well described by classical physics. Classical spacetime is smooth. However,
when curvature becomes extreme, near a singularity, then the quantum nature of
gravity cannot be ignored. Here, spacetime is no longer smooth, similar to a
carpet which looks smooth from afar but up-close is full of fibers and threads.
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- Thus, near the Big Bang singularity, the
structure of spacetime ceases to be smooth. Mathematical theorems suggest that
spacetime becomes overwhelmed by features of hooks, loops and bubbles. This
rapidly fluctuating situation is called “spacetime foam”.
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- In “spacetime foam”, causality does not
apply, because there are closed loops in spacetime where the future of an event
is also its past (so its outcome can also be its cause). The probabilistic
nature of quantum theory suggests that, when the probability distribution is
evenly spread out, all outcomes are equally possible and the comfortable notion
of causality we associate with a classical understanding of physics is lost.
-
- Therefore, if we go back in time, just
before we encounter the Big Bang singularity, we find ourselves entering an
epoch where the quantum effects of gravity are dominant and causality does not
apply. This is called the “Planck epoch”.
-
- Time ceases to be linear, going from the
past to the future, and instead becomes wrapped, chaotic and random. This means
the question "why did the Big Bang occur?" has no meaning, because
outside causality, “events do not need a cause to take place”.
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- In order to understand how physics works at
a singularity like the Big Bang, we need a theory for how gravity behaves
according to quantum theory. Unfortunately, we do not have one. There are a
number of efforts on this front like “loop quantum gravity” and “string
theory”.
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- However, these efforts are at best
incomplete, because the problem is notoriously difficult.
So how did our expanding and
largely classical universe ever escape from spacetime foam? This brings us to
“cosmic inflation”. This is defined as a period of accelerated expansion in the
early universe. It was first introduced by the Russian theoretical physicist
Alexei Starobinsky in 1980 and in parallel, that same year, by the American
physicist Alan Guth, coined the name.
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- “Inflation” makes the universe large and
uniform, according to observations. It also forces the universe to be spatially
flat, which is an otherwise unstable situation, but which has also been
confirmed by observations. Inflation
provides a natural mechanism to generate the primordial irregularities in the
density of the universe that are essential for structures such as galaxies and
galaxy clusters to form.
-
- Precision observations of the “cosmic
microwave background” in recent decades have spectacularly confirmed the
predictions of “inflation”. We also know that the universe can indeed undergo
accelerated expansion, because in the last few billion years it started doing
it again.
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- If the conditions for inflation arise (by
chance) in a patch of fluctuating spacetime, as can occur with spacetime foam,
then this region inflates and starts conforming to classical physics.
According to an idea first
proposed by the Russian-American physicist Andrei Linde, inflation is a
natural, and perhaps inevitable, consequence of chaotic initial conditions in
the early universe.
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- The point is that our classical universe
could have emerged from chaotic conditions, like those in spacetime foam, by
experiencing an initial boost of inflation. This would have set off the
expansion of the universe. In fact, the observations by astronomers of the CMB
suggest that the initial boost is explosive, since the expansion is exponential
during inflation.
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- The 14 billion-year story of our universe
begins with a cataclysmic explosion everywhere in space, which we call the Big
Bang. That much is beyond reasonable doubt. This explosion is really a period
of explosive expansion, which we call cosmic inflation. What happens before
inflation, though? Is it a spacetime singularity, is it spacetime foam? The
answer is largely unknown.
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- In fact, it might even be unknowable,
because there is a mathematical theorem which forbids us from accessing
information about the onset of inflation, much like the one that prevents us
from knowing about the interiors of black holes. So, from our point of view,
cosmic inflation is the Big Bang, the explosion that started it all.
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- November 5, 2024 BIG
BANG THEORY - what
started it all 4601
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--------------------- --- Thursday, November 7,
2024
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