[As we gain new knowledge, our scientific picture of how the
Universe works must evolve. This is a feature of the Big Bang, not a
bug.]
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THE BIG BANG NO LONGER MEANS WHAT IT USED TO  
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Ethan Siegel
August 24, 2022
Big Think [[link removed]]

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_ As we gain new knowledge, our scientific picture of how the
Universe works must evolve. This is a feature of the Big Bang, not a
bug. _

From a pre-existing state, inflation predicts that a series of
universes will be spawned as inflation continues, with each one being
completely disconnected from every other one. One of these "bubbles,"
where inflation ended is our Universe., Credit: Nicolle Rager Fuller

 

* The idea that the Universe had a beginning, or a "day without a
yesterday" as it was originally known, goes all the way back to
Georges Lemaître in 1927. 
* Although it's still a defensible position to state that the
Universe likely had a beginning, that stage of our cosmic history has
very little to do with the "hot Big Bang" that describes our early
Universe. 
* Although many laypersons (and even a minority of professionals)
still cling to the idea that the Big Bang means "the very beginning of
it all," that definition is decades out of date. Here's how to get
caught up.

If there’s one hallmark inherent to science, it’s that our
understanding of how the Universe works is always open to revision in
the face of new evidence. Whenever our prevailing picture of reality
— including the rules it plays by, the physical contents of a
system, and how it evolved from its initial conditions to the present
time — gets challenged by new experimental or observational data, we
must open our minds to changing our conceptual picture of the cosmos.
This has happened many times since the dawn of the 20th century, and
the words we use to describe our Universe have shifted in meaning as
our understanding has evolved.

Yet, there are always those who cling to the old definitions, much
like linguistic prescriptivists
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acknowledge that these changes have occurred. But unlike the evolution
of colloquial language, which is largely arbitrary, the evolution of
scientific terms must reflect our current understanding of reality.
Whenever we talk about the origin of our Universe, the term “the Big
Bang” comes to mind, but our understanding of our cosmic origins
have evolved tremendously since the idea that our Universe even had an
origin, scientifically, was first put forth. Here’s how to resolve
the confusion and bring you up to speed on what the Big Bang
originally meant versus what it means today.

Credit: British Broadcasting Company. Fred Hoyle was a regular on BBC
radio programs in the 1940s and 1950s, and one of the most influential
figures in the field of stellar nucleosynthesis. His role as the Big
Bang’s most vocal detractor, even after the critical evidence
supporting it had been discovered, is one of his longest-enduring
legacies.

The first time the phrase “the Big Bang” was uttered was over 20
years after the idea was first described. In fact, the term itself
comes from one of the theory’s greatest detractors: Fred Hoyle, who
was a staunch advocate of the rival idea of a Steady-State cosmology.
In 1949, he appeared on BBC radio
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advocated for what he called the perfect cosmological principle: the
notion that the Universe was homogeneous in both space _and time_,
meaning that any observer not only anywhere but _anywhen_ would
perceive the Universe to be in the same cosmic state. He went on to
deride the opposing notion as a “hypothesis that all matter of the
universe was created in one _Big Bang_ at a particular time in the
remote past,” which he then called “irrational” and claimed to
be “outside science.”

But the idea, in its original form, wasn’t simply that all of the
Universe’s matter was created in one moment in the finite past. That
notion, derided by Hoyle, had already evolved from its original
meaning. Originally, the idea was that the Universe _itself_, not
just the matter within it, had emerged from a state of non-being in
the finite past. And that idea, as wild as it sounds, was an
inevitable but difficult-to-accept consequence of the new theory of
gravity put forth by Einstein back in 1915: General Relativity.

Credit: Christopher Vitale of Networkologies and the Pratt Institute.
Instead of an empty, blank, three-dimensional grid, putting a mass
down causes what would have been ‘straight’ lines to instead
become curved by a specific amount. In General Relativity, we treat
space and time as continuous, but all forms of energy, including but
not limited to mass, contribute to spacetime curvature. The deeper you
are in a gravitational field, the more severely all three dimensions
of your space is curved, and the more severe the phenomena of time
dilation and gravitational redshift become.

When Einstein first cooked up the general theory of relativity, our
conception of gravity forever shifted from the prevailing notion of
Newtonian gravity. Under Newton’s laws, the way that gravitation
worked was that any and all masses in the Universe exerted a force on
one another, instantaneously across space, in direct proportion to the
product of their masses and inversely proportional to the square of
the distance between them. But in the aftermath of his discovery of
special relativity, Einstein and many others quickly recognized that
there was no such thing as a universally applicable definition of what
“distance” was or even what “instantaneously” meant with
respect to two different locations.

With the introduction of Einsteinian relativity — the notion that
observers in different frames of reference would all have their own
unique, equally valid perspectives on what distances between objects
were and how the passage of time worked — it was only almost
immediate that the previously absolute concepts of “space” and
“time” were woven together into a single fabric: spacetime. All
objects in the Universe moved through this fabric, and the task for a
novel theory of gravity would be to explain how not just masses, but
all forms of energy, shaped this fabric that underpinned the Universe
itself.

Credit: E. Siegel/Beyond the Galaxy. If you begin with a bound,
stationary configuration of mass, and there are no non-gravitational
forces or effects present (or they’re all negligible compared to
gravity), that mass will always inevitably collapse down to a black
hole. It’s one of the main reasons why a static, non-expanding
Universe is inconsistent with Einstein’s General Relativity.

Although the laws that governed how gravitation worked in our Universe
were put forth in 1915, the critical information about how our
Universe was structured had not yet come in. While some astronomers
favored the notion that many objects in the sky were actually
“island Universes” that were located well outside the Milky Way
galaxy, most astronomers at the time thought that the Milky Way galaxy
represented the full extent of the Universe. Einstein sided with this
latter view, and — thinking the Universe was static and eternal —
added a special type of fudge factor into his equations: a
cosmological constant.

Subscribe to Starts With A Bang
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Our mission: to answer, scientifically, the biggest questions of
all. What is our universe made of? How did it become the way it is
today? Where did everything come from? What is the ultimate fate of
the cosmos? 

For countless generations, these were questions without resolutions.
Now, for the first time in history, we have scientific answers. Starts
With A Bang, written by Dr. Ethan Siegel, brings these stories — of
what we know and how we know it — directly to you.

Although it was mathematically permissible to make this addition, the
reason Einstein did so was because without one, the laws of General
Relativity would ensure that a Universe that was evenly, uniformly
distributed with matter (which ours seemed to be) would be unstable
against gravitational collapse. In fact, it was very easy to
demonstrate that any initially uniform distribution of motionless
matter, regardless of shape or size, would inevitably collapse into a
singular state under its own gravitational pull. By introducing this
extra term of a cosmological constant, Einstein could tune it so that
it would balance out the inward pull of gravity by proverbially
pushing the Universe out with an equal and opposing action.

Credit: E. Hubble; R. Kirshner, PNAS, 2004.  Edwin Hubble’s
original plot of galaxy distances versus redshift (left), establishing
the expanding Universe, versus a more modern counterpart from
approximately 70 years later (right). In agreement with both
observation and theory, the Universe is expanding, and the slope of
the line relating distance to recession speed is a constant.

Two developments — one theoretical and one observational — would
quickly change this early story that Einstein and others had told
themselves.

* In 1922, Alexander Friedmann worked out, fully, the equations that
governed a Universe that was isotropically (the same in all
directions) and homogeneously (the same in all locations) filled with
any type of matter, radiation, or other form of energy. He found that
such a Universe would never remain static, not even in the presence of
a cosmological constant, and that it must either expand or contract,
dependent on the specifics of its initial conditions.
* In 1923, Edwin Hubble became the first to determine that the
spiral nebulae in our skies were not contained within the Milky Way,
but rather were located many times farther away than any of the
objects that comprised our home galaxy. The spirals and ellipticals
found throughout the Universe were, in fact, their own “island
Universes,” now known as galaxies, and that moreover — as had
previously been observed by Vesto Slipher — the vast majority of
them appeared to be moving away from us at remarkably rapid speeds.

In 1927, Georges Lemaître became the very first person to put these
pieces of information together, recognizing that the Universe today is
expanding, and that if things are getting farther apart and less dense
today, then they must have been closer together and denser in the
past. Extrapolating this back all the way to its logical conclusion,
he deduced that the Universe must have expanded to its present state
from a single point-of-origin, which he called either the “cosmic
egg” or the “primeval atom.”

Credit: Public Domain. This image shows Catholic priest and
theoretical cosmologist Georges Lemaître at the Catholic University
of Leuven, ca. 1933. Lemaître was among the first to conceptualize
the Big Bang as the origin of our Universe within the framework of
General Relativity, even though he didn’t use that name himself.

This was the original notion of what would grow into the modern theory
of the Big Bang: the idea that the Universe had a beginning, or a
“day without yesterday.” It was not, however, generally accepted
for some time. Lemaître originally sent his ideas to Einstein,
who infamously dismissed Lemaître’s work
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responding, “Your calculations are correct, but your physics is
abominable.”

Despite the resistance to his ideas, however, Lemaître would be
vindicated by further observations of the Universe. Many more galaxies
would have their distances and redshifts measured, leading to the
overwhelming conclusion the Universe was and still is expanding,
equally and uniformly in all directions on large cosmic scales. In the
1930s, Einstein conceded, referring to his introduction of the
cosmological constant in an attempt to keep the Universe static as his
“greatest blunder.”

However, the next great development in formulating what we know of as
the Big Bang wouldn’t come until the 1940s, when George Gamow —
perhaps not so coincidentally, an advisee of Alexander Friedmann —
came along. In a remarkable leap forward, he recognized that the
Universe was not only full of matter, but also radiation, and that
radiation evolved somewhat differently from matter in an expanding
Universe. This would be of little consequence today, but in the early
stages of the Universe, it mattered tremendously.

Credit: E. Siegel/Beyond the Galaxy. While matter (both normal and
dark) and radiation become less dense as the Universe expands owing to
its increasing volume, dark energy, and also the field energy during
inflation, is a form of energy inherent to space itself. As new space
gets created in the expanding Universe, the dark energy density
remains constant. Note that individual quanta of radiation are not
destroyed, but simply dilute and redshift to progressively lower
energies, stretching to longer wavelengths and lower energies as space
expands.

Matter, Gamow realized, was made up of particles, and as the Universe
expanded and the volume that these particles occupied increased, the
number density of matter particles would drop in direct proportion to
how the volume grew.

But radiation, while also made up of a fixed number particles in the
form of photons, had an additional property: the energy inherent to
each photon is determined by the photon’s wavelength. As the
Universe expands, the wavelength of each photon gets lengthened by the
expansion, meaning that the amount of energy present in the form of
radiation decreases faster than the amount of energy present in the
form of matter in the expanding Universe.

But in the past, when the Universe was smaller, the opposite would
have been true. If we were to extrapolate backward in time, the
Universe would have been in a hotter, denser, more radiation-dominated
state. Gamow leveraged this fact to make three great, generic
predictions about the young Universe.

* At some point, the Universe’s radiation was hot enough so that
every neutral atom would have been ionized by a quantum of radiation,
and that this leftover bath of radiation should still persist today at
only a few degrees above absolute zero.
* At some even earlier point, it would have been too hot to even
form stable atomic nuclei, and so an early stage of nuclear fusion
should have occurred, where an initial mix of protons-and-neutrons
should have fused together to create an initial set of atomic nuclei:
an abundance of elements that predates the formation of atoms.
* And finally, this means that there would be some point in the
Universe’s history, after atoms had formed, where gravitation pulled
this matter together into clumps, leading to the formation of stars
and galaxies for the first time.

Credit: S.C. Djorgovski et al., Caltech; Caltech Digital Media Center.
Schematic diagram of the Universe’s history, highlighting
reionization. Before stars or galaxies formed, the Universe was full
of light-blocking, neutral atoms that formed back when the Universe
was ~380,000 years old. Most of the Universe doesn’t become
reionized until 550 million years afterwards, with some regions
achieving full reionization earlier and others later. The first major
waves of reionization begin happening at around ~200 million years of
age, while a few fortunate stars may form just 50-to-100 million years
after the Big Bang. With the right tools, like the JWST, we hope to
reveal the earliest galaxies of all.

These three major points, along with the already-observed expansion of
the Universe, form what we know today as the four cornerstones of the
Big Bang. Although one was still free to extrapolate the Universe back
to an arbitrarily small, dense state — even to a singularity, if
you’re daring enough to do so — that was no longer the part of the
Big Bang theory that had any predictive power to it. Instead, it was
the emergence of the Universe from a hot, dense state that led to our
concrete predictions about the Universe.

Over the 1960s and 1970s, as well as ever since, a combination of
observational and theoretical advances unequivocally demonstrated the
success of the Big Bang in describing our Universe and predicting its
properties.

* The discovery of the cosmic microwave background and the subsequent
measurement of its temperature and the blackbody nature of its
spectrum eliminated alternative theories like the Steady State model.
* The measured abundances of the light elements throughout the
Universe verified the predictions of Big Bang nucleosynthesis, while
also demonstrating the need for fusion in stars to provide the heavy
elements in our cosmos.
* And the farther away we look in space, the less grown-up and
evolved galaxies and stellar populations appear to be, while the
largest-scale structures like galaxy groups and clusters are less rich
and abundant the farther back we look.

The Big Bang, as verified by our observations, accurately and
precisely describes the emergence of our Universe, as we see it, from
a hot, dense, almost-perfectly uniform early stage.

But what about the “beginning of time?” What about the original
idea of a singularity, and an arbitrarily hot, dense state from which
space and time themselves could have first emerged?

Credit: NASA/CXC/M. Weiss. 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. However, the Big Bang was not an explosion, and
cosmic expansion is very different from that idea.

That’s a different conversation, today, than it was back in the
1970s and earlier. Back then, we knew that we could extrapolate the
hot Big Bang back in time: back to the first fraction-of-a-second of
the observable Universe’s history. Between what we could learn from
particle colliders and what we could observe in the deepest depths of
space, we had lots of evidence that this picture accurately described
our Universe.

But at the absolute earliest times, this picture breaks down. There
was a new idea — proposed and developed in the 1980s — known as
cosmological inflation, that made a slew of predictions that
contrasted with those that arose from the idea of a singularity at the
start of the hot Big Bang. In particular, inflation predicted:

* A curvature for the Universe that was indistinguishable from flat,
to the level of between 99.99% and 99.9999%; comparably, a singularly
hot Universe made no prediction at all.
* Equal temperatures and properties for the Universe even in
causally disconnected regions; a Universe with a singular beginning
made no such prediction.
* A Universe devoid of exotic high-energy relics like magnetic
monopoles; an arbitrarily hot Universe would possess them.
* A Universe seeded with small-magnitude fluctuations that were
almost, but not perfectly, scale invariant; a non-inflationary
Universe produces large-magnitude fluctuations that conflict with
observations.
* A Universe where 100% of the fluctuations are adiabatic and 0% are
isocurvature; a non-inflationary Universe has no preference.
* A Universe with fluctuations on scales larger than the cosmic
horizon; a Universe originating solely from a hot Big Bang cannot have
them.
* And a Universe that reached a finite maximum temperature that’s
well below the Planck scale; as opposed to one whose maximum
temperature reached all the way up to that energy scale.

The first three were post-dictions of inflation; the latter four were
predictions that had not yet been observed when they were made. On all
of these accounts, the inflationary picture has succeeded in ways that
the hot Big Bang, without inflation, has not.

Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task
Force on CMB research. The quantum fluctuations that occur during
inflation get stretched across the Universe, and when inflation ends,
they become density fluctuations. This leads, over time, to the
large-scale structure in the Universe today, as well as the
fluctuations in temperature observed in the CMB. New predictions like
these are essential for demonstrating the validity of a proposed
fine-tuning mechanism, and to test (and potentially rule out)
alternatives.

During inflation, the Universe must have been devoid of
matter-and-radiation and instead contained some sort of energy —
whether inherent to space or as part of a field — that didn’t
dilute as the Universe expanded. This means that inflationary
expansion, unlike matter-and-radiation, didn’t follow a power law
that leads back to a singularity but rather is exponential in
character. One of the fascinating aspects about this is that something
that increases exponentially, even if you extrapolate it back to
arbitrarily early times, even to a time where _t_ → -∞, it never
reaches a singular beginning.

Now, there are many reasons to believe that the inflationary state
wasn’t one that was eternal to the past, that there might have been
a pre-inflationary state that gave rise to inflation, and that,
whatever that pre-inflationary state was, perhaps it did have a
beginning. There are theorems that have been proven and loopholes
discovered to those theorems, some of which have been closed and some
of which remain open, and this remains an active and exciting area of
research.

Credit: E. Siegel. Blue and red lines represent a “traditional”
Big Bang scenario, where everything starts at time t=0, including
spacetime itself. But in an inflationary scenario (yellow), we never
reach a singularity, where space goes to a singular state; instead, it
can only get arbitrarily small in the past, while time continues to go
backward forever. Only the last minuscule fraction of a second, from
the end of inflation, imprints itself on our observable Universe
today.

But one thing is for certain.

Whether there was a singular, ultimate beginning to all of existence
or not, it no longer has anything to do with the hot Big Bang that
describes our Universe from the moment that:

* inflation ended,
* the hot Big Bang occurred,
* the Universe became filled with matter and radiation and more,
* and it began expanding, cooling, and gravitating,

eventually leading to the present day. There are still a minority of
astronomers, astrophysicists and cosmologists who use “the Big
Bang” to refer to this theorized beginning and emergence of
time-and-space, but not only is that not a foregone conclusion
anymore, but it doesn’t have anything to do with the hot Big Bang
that gave rise to our Universe. The original definition of the Big
Bang has now changed, just as our understanding of the Universe has
changed. If you’re still behind, that’s ok; the best time to catch
up is always right now.

_Additional recommended reading:_

* Ask Ethan: Do we know why the Big Bang really happened?
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for cosmic inflation)
* Surprise: the Big Bang isn’t the beginning of the universe
anymore
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a “singularity” is no longer necessarily a given)

_ETHAN SIEGEL, host of popular podcast "Starts with a Bang!" is a
Ph.D. astrophysicist and author of "Starts with a Bang!" He is a
science communicator, who professes physics and astronomy at various
colleges. He has won numerous awards for science writing since 2008
for his blog, including the award for best science blog by the
Institute of Physics. His two books "Treknology: The Science of Star
Trek from Tricorders to Warp Drive"
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the Galaxy: How humanity looked beyond our Milky Way and discovered
the entire Universe"
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available for purchase at Amazon. Follow him on
Twitter @startswithabang [[link removed]]. _

_BIG THINK. Our mission is to make you smarter, faster. At Big
Think, we introduce you to the brightest minds and boldest ideas of
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understand our ever-changing world. Subscribe.
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