In Defense of the Big Bang
© 1996 Neil de Grasse Tyson
Featured in Natural History Magazine
December 1996 / January 1997
What, you might ask, could possibly induce a
rational astrophysicist to believe that all the matter, energy, and space
of the universe began fifteen billion years ago in a primeval fireball
packed into a volume smaller than a marble that has been expanding ever
since? The answer is simple: regardless of what you may have
read or heard, the big bang is supported by a preponderance of evidence
and has become the most successful theory ever put forth for the origin
and evolution of the universe.
Scientific evidence in support of a theory
sometimes takes you places where your senses have never been. Common
sense is that human ability to assess a situation you have never seen before
by invoking life experiences derived from your five senses. But twentieth-century
science has largely been built upon data that was, and continues to be,
collected with all manner of tools that enable us to see the universe in
decidedly uncommon ways. As a consequence, while we have always required
that a theory make mathematical sense, we no longer require that a theory
make common sense. We simply demand that it be consistent with the
results of observations and experiments. This posture has enabled
profound, yet remarkably counterintuitive branches of physics, such as
relativity, quantum mechanics, and big bang cosmology to arise.
Of all the theories about how the physical
world works, the general public seems to be most intrigued by the big bang.
Who wouldn't? Ideas about the origin of things have always made fascinating
science. But I have found some people that vehemently oppose the
big bang while being generally uninformed about its fundamental tenets.
A well-constructed theory should explain some of what is not understood
and, more importantly. predict previously unknown phenomena that can be
tested. A successful theory is one where experiments consistently
confirm its predictions.
Some like to claim that the big bang is
"just a theory" and should therefore be discounted. Don't be fooled.
The beginning of the twentieth century saw the end of labeling successful
theories as "laws." This change of vocabulary came
when new experimental domains revealed the predictions of previous physical
laws to be incomplete. The change was the physicist's humble recognition
that data from newer and better equipment might provide a deeper realization
of the physical world. This is why pre-1900 we had Kepler's
laws of planetary motion, Newton's laws of gravity, and the laws
of thermodynamics, whereas after 1900 we have Einstein's theory
of relativity, quantum theory, big bang theory, and so forth.
Confidence in big-bang cosmology is derived
from the strengths of many arguments. Let us start with Edwin Hubble's
1929 observation that we live in an expanding universe, where distant galaxies
recede from us faster than the near ones in direct proportion to their
distances. Further support came from Albert Einstein's theory of
gravity, better known as the general theory of relativity, which predicted
an expanding universe as one of its solutions with the precise expansion
pattern found by Hubble. Since Einstein's theory preceded Hubble's
discovery (by thirteen years), Einstein cannot be accused of putting forth
an after-the-fact explanation.
For any theory, one should not hesitate
to question every possible assumption, no matter how basic they are.
If you happen to have a gripe with the claim that objects with high velocity
of recession, are farther away than objects with low velocity of recession
then consider the existence of gravitational lenses as a simple test-case.
As first predicted by Einstein, the gravity of a high-mass foreground object
can distort space in its vicinity so that an object which, by chance, falls
along the line of sight in the background , can look as though it is split
into two or more images. These optical antics have been observed
in dozens of galaxies all around the sky and the"lensed"
object (presumed to be in the background simply because it was the one
that got lensed ) always has a higher recession velocity than the object
whose gravity is serving as the lens itself.
Perhaps it's some kind of an illusion that very
distant galaxies are receding from us at very high speeds. If indeed
they are moving at very high speeds then they ought to measurably exhibit
the effects of "time dilation" predicted in Einstein's theory
of relativity, where time ticks much more slowly for an object that you
observe to have high velocity. Recently, supernovae discovered in
distant galaxies have been found to take more time to explode and decline
in luminosity than counterpart supernovae in nearby galaxies. That
extra time happens to be precisely what you would expect from their extreme
velocity of recession and the consequent effects of Einstein's relativity.
The most powerful supporting argument for
the big bang derives from the "cosmic microwave background".
Shortly after the Second World War, and shortly after the notion of a hot,
explosive origin for the universe was proposed by the physicist George
Gamow, the physicists Ralph Alpher and Robert Herman invoked simple principles
of thermodynamics and particle physics to infer that the density of matter
and energy of the universe must have been higher in the past, concluding
that there should be a leftover signal from an earlier time, when the ambient
temperature of the universe was thousands of degrees. That leftover
signal, by virtue of the expanding universe, should have cooled appreciably
and would appear today as an omni-directional bath of microwave energy
with a characteristic temperature of a few degrees on the Kelvin absolute
temperature scale. In 1965 a part of this background signal serendipitously
revealed itself in data obtained by the microwave antennae of two Bell
Labs physicists, Arno Penzias and Robert Wilson, for which they were jointly
awarded the 1978 Nobel prize in physics.
If you have a gripe with the claim that
some accidentally discovered microwaves are the cooled remnant of a youthful,
hot universe, then consider that the big bang predicts a specific mixture
of energy for this bath of microwaves that characterizes a single temperature.
By similar reasoning, the specific mixture of energy emitted by the Sun
(including the relative amounts of infrared, visible and ultraviolet light),
characterizes a single temperature (6000 kelvins) at its surface.
In 1990, the COBE satellite (COsmic Background Explorer) measured this
background and indicated a single temperature (2.726 kelvins) to an accuracy
of two-tenths of one percent.
You might be skeptical about whether this single-temperature
assortment of microwaves actually came from the early universe. You
might prefer to think they were created by your neighbor's microwave oven
or a police radar gun or by some microwave-emitting wall of interstellar
material nearby in space. But we know that the gravity of galaxy
clusters slightly reduces the energy of light that passes through them.
And when we look for what the microwave background does in the line of
sight to these distant clusters we see a slight drop in energy, implying
that the microwave background indeed hails from beyond these clusters and
not in front of them.
You may not be convinced that the universe was
hotter in the past than it is today, as it must have been in the big bang
picture. Consider distant galaxies, which, because of the light travel
time between the galaxy and us, we see not as they are but as they once
were. If big bang cosmology is correct, these distant galaxies should
be bathed in a hotter cosmic background than what is measured in the present.
Sensitive measurements of molecules that react differently to different
background temperatures have allowed us to infer a temperature for the
cosmic background from distant galaxies that is in precise accord with
the predicted temperature of the universe at the time the light that we
measured left these galaxies.
Just for fun, let's turn back the big bang clock,
and use current laws of physics to extrapolate the behavior of the universe
to a time when it was much smaller, denser, and hotter - when the
background was upwards of a trillion degrees. (Our current theories
of physics actually allow us to describe the behavior of the universe starting
from the first 0.0000000000000000000000000000000000000-000001 seconds
of its existence all the way up to 15 billion years and beyond. Times
earlier than this 10-43 seconds have no meaning in quantum mechanics.)
At these early times and high temperatures, all atoms were broken apart
into their component nuclear particles. Combining all that we know
of quantum mechanics, particle physics, and all we have learned from busting
atoms to smithereens in particle accelerators, we conclude that as the
cosmic soup expanded and cooled, nuclear particles recombined to make a
specific and predictable assortment of atoms: the universe was born
with 75 percent of its mass as hydrogen and about 25 percent as helium.
These are bold extrapolations, but surveys of the most helium-deficient
galaxies (those that have undergone very little star formation and hence
suffered very little contamination) routinely find between 22 and 27 percent
helium, in good agreement with big bang predictions.
A few other light elements are predicted to have
formed in trace amounts during the first several moments of the universe.
Among these are "heavy" hydrogen (which is simply a proton and a neutron),
"light" helium (which is simply helium that is missing a neutron from its
nucleus), and lithium (the third lightest element on the periodic table
of elements). The measured quantities of these light elements in
the universe are also consistent with the predictions from the big bang.
We didn't just make this stuff up. It represents
an unprecedented marriage of astrophysics and particle physics where a
coherent cosmic picture has emerged from a minimum of assumptions that
tells us the galaxy velocities are real, the galaxy distances are real,
the expanding universe is real, relativity is real, quantum mechanics is
real, and the big bang is real. Whenever different sub-branches of
a science support the same theory then the confidence you bestow upon the
theory is greatly enhanced.
But alas, all is not perfect in paradise.
There remains a few holes in big bang theory.
Most importantly, the density of mass in the
universe today implies an initial value that is remarkably close to the
critical density, which is the density that packs just enough mass for
the universe to live at the boundary between one that will ultimately recollapse
and one that will expand forever. The fine- tuning that this requires
among the values for many of the cosmological parameters in the early universe
could not have happened randomly.
And going deeper than the simple extrapolations
of the big bang we find that the microwave background is far too uniform
from one patch of the sky to the next to have emerged from the conditions
thought to have been present in the early universe.
Unfortunately, the early, rapid expansion of
the universe does not leave enough time for the galaxies to form as we
think they should form; and the big bang cannot tell us what happened
before 10-43 seconds, or for that matter, what happened before zero seconds
- or why the laws of physics are what they are.
Do we throw away the big bang along with the
bath water because of these complications? Or do we retain the big
bang's successful predictions and see if there is room to modify the theory's
details in an attempt to solve these problems? These sorts of questions
have arisen before. In the mid-sixteenth century, the Polish astronomer
Nicolaus Copernicus proposed a model of the known universe with the Sun
as the center of all motion rather than Earth. This heliocentric
model was much, much simpler than the competing geocentric model because
it removed the need for complex epicycles to account for the motions of
the planets in the sky, especially during their occasional retrograde motion.
But there was a problem. The predicted paths of the planets in the
heliocentric model continually deviated from the actual paths of the planets
in the sky. Should Copernicus have therefore discarded the entire
idea of a Sun-centered universe, or should he have modified some of the
model's details? Copernicus' heliocentric view was, of course, basically
correct. The problems arose because he naively assumed that the planets
orbited the Sun in perfect circles rather than in ellipses, and of course,
the concept of gravity was not yet invented. It would be two hundred
years before Isaac Newton's universal law of gravitation supplied a bigger
picture that modified and completely subsumed Copernicus' view of the world.
Progress has already been made to resolve some
of the problems with the big bang model. The most significant modification
is known as inflationary cosmology, where the energetics of the very early
universe passes through a phase that spontaneously triggers an period of
extremely rapid expansion. Inflation naturally accounts for what
was thought to be an embarrassingly fine- tuned critical
density. It also allows the cosmic microwave background to be as
uniform as it is measured to be. Introduced in the early 1980s
by the American physicist Alan Guth, inflation is a natural consequence
of the principles of quantum mechanics when applied to the fabric of space
and time in the early universe and thus has no household analog.
Inflation's main prediction is that the universe was born with its mass
density equal to the critical value and continues today have the critical
mass density. Current observations have recovered anywhere from 20
to 40 percent of the mass necessary to reach the critical density.
Inflation enthusiasts are fervently looking for the rest.
One class of inflationary theories describes
a mega-universe with multiple areas of expansion where each region looks
like a big bang universe from within, and where different regions of expansion
can sustain laws of physics that differ from the ones we know. If
this model can be tested and supported then inflation will have subsumed
the entire big bang into a larger cosmological picture.
If you choose to discard the big bang entirely
then step lightly, you will be forfeiting an impressive array of successful
predictions - far more than most theories-in-progress enjoy.
Nearly everyone in the community of astrophysicists has chosen to work
with it, recognizing that our efforts may lead to an even deeper understanding
of the universe where the big bang becomes the core idea of something even
Neil de Grasse Tyson, an astrophysicist, is the Frederick P. Rose Director
of the Hayden Planetarium at the American Museum of Natural History.
He is also a research scientist at Princeton University.
This article is available here for UGA GEOL 1122 students by the very kind permission of Dr. Tyson.
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