- 4593 - BIG BANG THEORY - that started everything? 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
------------------------------- 4593
- BIG BANG
THEORY - that started everything?
-
- How did everything begin? It's a question
that humans have pondered for thousands of years. Over the last century or so,
science has homed in on an answer: the Big Bang.
-
- The Big Bang 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. The early universe was extraordinarily
hot and extremely dense. But how do we know this happened?
-
- Let's look first at the evidence. 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 universe, the
expansion would reverse and the galaxies would fall on top of each other 14
billion years ago.
-
- The English astronomer Fred Hoyle
sarcastically dismissed the hypothesis as a "Big Bang" during an
interview with BBC radio on March 28, 1949.
Then, 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.
-
- 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. 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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”.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- But mathematics suggests there are places in
the universe where the curvature of spacetime becomes unlimited. These
locations are known as “singularities”. One such example can be found at the
center of a black hole. At these places, the theory of general relativity
breaks down.
-
- 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”.
-
- 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 “onset of time”.
-
- 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”.
-
- 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.
-
- 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.
-
- What about gravity? 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, but gnarly, similar to a carpet which looks smooth from afar but
up-close is full of fibers and threads.
-
- Thus, near the Big Bang singularity, the
structure of spacetime ceases to be smooth. Mathematical theorems suggest that
spacetime becomes overwhelmed by "gnarly" features: hooks, loops and
bubbles. This rapidly fluctuating situation is called “spacetime foam”.
-
- 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.
-
- 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”, with its various incarnations.
-
- However, these efforts are at best
incomplete, because the problem is notoriously difficult. This means that
spacetime foam has a powerful mystique.
So how did our expanding and largely classical universe ever escape from
spacetime foam? This brings us to “cosmic inflation”.
-
- Inflation
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,
who coined the name.
-
- 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. Moreover, 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.
-
- What does this have to do with spacetime
foam? 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.
-
- 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.
-
- In March 20 of 2014, Alan Guth explained it
succinctly: "I usually describe inflation as a theory of the 'bang' of the
Big Bang: It describes the propulsion mechanism that we call the Big
Bang."
-
- From our point of view, cosmic inflation is
the Big Bang, the explosion that started it all.
-
-
October 31, 2024 BIG BANG
THEORY - that started everything? 4593
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