Evidence for the Big Bang

The evolution of the world can be compared to a display of fireworks that has just ended, some few red wisps, ashes, and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origin of the worlds.” – Fr. Georges Lemaitre

Since the ancient astronomers, humans have wondered about the origins of the universe.  For most of history, mythology filled the void in our knowledge.  Then with Issac Newton, scientists began to assume the universe was infinite in both time and space.  The concept of a universe that had a discrete origin was considered religious and not scientific.  During the 20th century, dramatic advancements in both theory and observation provided a definitive explanation how the universe originated and evolved.  Most people I talk to, especially in America, are under the impression the Big Bang is just a theory without any evidence.  Nothing could be further from the truth.  In fact, every time you drink a glass of water, you are drinking the remnants of the Big Bang.

In 1916, Einstein published his general theory of relativity.  Rather than viewing gravity as an attractive force between bodies of mass, relativity describes gravity as mass bending the fabric of space-time.  Think of a flat trampoline with nothing on it.  If you roll a marble across the trampoline, it moves in a straight path.  Now put a bowling ball on the trampoline, the marble’s path is deflected by the bend in the trampoline.  This is analogous to the Sun bending space-time deflecting the paths of planets.  Once Einstein finished up on relativity, he endeavored to build models of the universe with his new theory.  These models produced one puzzling feature.

The equations describing the universe with relativity produced the term dr/dt.  The radius of the universe could expand or contract as time progresses.  If you introduced matter into the model, gravity would cause space-time and the universe itself to contract.  That didn’t seem to reflect reality, and Einstein was still operating with the Newtonian notion of an infinite, unchanging universe.  To check the contraction of the universe, Einstein included a cosmological constant to relativity to offset the force of gravity precisely.  By doing this, Einstein missed out on one of the great predictions made by his theory.

Balancing forces are not unheard of in nature.  In a star like the Sun, the inward force of gravity is offset by the outward force of gas as it moves from high pressure in the core to lower pressure regions towards the surface.  This is referred to as hydrostatic equilibrium and prevents the Sun from collapsing upon itself via gravity to form a black hole.  Einstein’s cosmological constant served the same purpose by preventing the universe as a whole from collapsing into a black hole via gravity.  However, during the 1920’s, a Catholic priest who was also a mathematician and astrophysicist, would provide a radical new model to approach this problem.

Georges Lemaitre had a knack for being where the action was.  As a Belgian artillery officer, Lemaitre witnessed the first German gas attack at Ypres in 1916.  Lemaitre was spared as the wind swept the gas towards the French sector.  After the war, Lemaitre would both enter the priesthood and receive PhD’s in mathematics and astronomy.  The math background provided Lemaitre with the ability to study and work on relativity theory.  The astronomy background put Lemaitre in contact with Arthur Eddington and Harlow Shapley, two of the most prominent astronomers of the time.  This would give Lemaitre a key edge in understanding both current theory and observational evidence.

It’s hard to imagine, but less than 100 years ago it was thought the Milky Way was the whole of the universe.  A new telescope, the 100-inch at Mt. Wilson, would provide the resolution power required to discern stars in other galaxies previously thought to be spiral clouds within the Milky Way.  One type of star, Cepheid variables, whose period of brightness correlates with its luminosity, provided a standard candle to measure galactic distances.  It was Edwin Hubble at Mt. Wilson who made this discovery.  Besides greatly expanding the size of the known universe, Hubble’s work unveiled another key aspect of space.

Edwin Hubble, Albert Einstein, & Fr. Georges Lemaitre. Credit: American Institute of Physics.

When stars and galaxies recede from Earth, their wavelengths of light are stretched out and move towards the red end of the spectrum.  This is akin to the sound of a car moving away from you.  Sound waves are stretched longer resulting in a lower pitch.  What Hubble’s work revealed was galaxies were moving away from each other.  Hubble was cautious in providing a rational for this.  However, Fr. Lemaitre had the answer.  It wasn’t so much galaxies were moving away as space was expanding between the galaxies as allowed by relativity theory.  Lemaitre also analyzed Hubble’s data to determine that the more distant a galaxy was, the greater its velocity moving away from us.  Lemaitre published this result in an obscure Belgian journal.  Hubble would independently publish the same result a few years later and received credit for what is now known as Hubble’s law.  This law equates recessional velocity to a constant (also named after Hubble) times distance.

As space is expanding between each object and in each direction, the recession velocity is greater the more distant an object is. Credit: OpenStax Astronomy/CC 4.0.

It would require more resolving power to determine the final value of the Hubble constant.  In fact, it took Hubble’s namesake, the Hubble Space Telescope to pin down the value which also provides the age of the universe.

Hubble’s original plot of recession velocity vs distance. The more distant a galaxy, the faster it is moving away from us. This is indicative of an expanding universe. Credit: doi: 10.1073/pnas.15.3.168 PNAS March 15, 1929 vol. 15 no. 3 168-173

In the meantime, the debate on the origin of the universe still needed to be settled.  Lemaitre favored a discrete beginning to the universe that evolved throughout its history.  Specifically, Lemaitre felt vacuum energy would cause the expansion of the universe to accelerate in time and thus, kept Einstein’s cosmological constant, albeit with a different value to speed up the expansion.  Einstein disagreed and thought the cosmological constant was no longer required.  By 1931, Einstein conceded the universe was expanding, but not accelerating as Lemaitre thought.  A decade later, the most serious challenge to the Big Bang theory emerged.

The label Big Bang was pinned on Lemaitre’s theory derisively by Fred Hoyle of Cambridge, who devised the Steady State theory.  This theory postulated an expanding universe, but the expansion was generated by the creation of new hydrogen.  Hoyle scored points with the discovery that stellar nucleosynthesis created the elements from carbon to iron via fusion processes.  Although Hoyle proved the Big Bang was not required to form these heavy elements, he still could not provide an answer to how hydrogen was created.  It would take some modifications to the Big Bang model to challenge Hoyle’s Steady State model.

Fred Hoyle and Fr. Georges Lemaitre. Credit: St. Edmond’s College/University of Cambridge.

During the 1940’s, George Gamow proposed a hot Big Bang as opposed to the cold Big Bang of Georges Lemaitre.  In Gamow’s model, the temperature of the universe reaches 1032 K during the first second of existence.  Gamow was utilizing advancements in quantum mechanics made after Lemaitre proposed his original Big Bang model.  Gamow’s model had the advantage over Hoyle’s Steady State model as it could explain the creation of hydrogen and most of the helium in the universe.  The hot Big Bang model had one additional advantage, it predicted the existence of a background microwave radiation emanating from all points in the sky and with a blackbody spectrum.

A blackbody is a theoretical construct.  It is an opaque object that absorbs all radiation (hence, it is black) and remits it as thermal radiation.  The key here is to emit blackbody radiation, an object has to be dense and hot.  A steady state universe would not emit blackbody radiation whereas a big bang universe would in its early stages.  During the first 380,000 years of its existence, a big bang universe would be a small, hot, and opaque.  By the time this radiation reached Earth some 13 billion years later, the expansion of the universe would stretch out these radiation wavelengths into the microwave range.  This stretching would correlate to a cooling of the blackbody radiation somewhere between 0 and 5 K or just 5 degrees above absolute zero.  Detection of this radiation, called the Cosmic Microwave Background (CMB) would resolve the Big Bang vs. Steady State debate.

In 1964, Arno Penzias and Robert Wilson were using the 20-foot horn antenna at Bell Labs to detect extra-galactic radio sources.  Regardless of where the antenna was pointed, they received noise correlating to a temperature of 2.7 K.  Cosmology was still a small, somewhat insular field separate from the rest of astronomy.  Penzias and Wilson did not know how to interpret this noise and made several attempts to rid themselves of it, including two weeks cleaning out pigeon droppings from the horn antenna.  Finally, they placed a call to Princeton University where they reached Robert Dicke, who had been building his own horn antenna to detect the CMB.  When the call ended, Dicke turned to his team and said:

“Boys, we’ve been scooped”

Actually, the first time the CMB was detected was in 1941 by Andrew McKeller but the theory to explain what caused it was not in place and the finding went forgotten.  Penzias and Wilson published their discovery simultaneously with Robert Dicke providing the theory explaining this was proof that the young universe was in a hot, dense, state after its origin.  Georges Lemaitre was told of the confirmation of the Big Bang a week before he passed away.  Penzias and Wilson were awarded the Nobel in 1978.  Back at Cambridge, Fred Hoyle refused to concede the Steady State theory was falsified until his death in 2001.  Some believe this refusal, among other things, cost Hoyle the Nobel in 1983 when it was awarded for the discovery of nucleosynthesis.  Hoyle was passed in favor of his junior investigator, Willy Fowler.

It would take 25 more years before another mystery of the CMB was solved.  The noise received by Penzias and Wilson was uniform in all directions.  For the stars and galaxies to form, some regions of the CMB had to be cooler than others.  This was not a failure on Penzias and Wilson’s part, but better equipment with higher resolution capabilities were required to detect the minute temperature differences.  In 1989, the COBE probe, headed by George Smoot, set out to map these differences in the CMB.  The mission produced the image below.  The blue regions are 0.00003 K cooler than the red regions, just cold enough for the first stars and galaxies to form.  This map is a representation of the universe when it was 380,000 years old.

The oval represents a 3-D spherical map of the universe projected into 2-D by cutting from one pole to the other. Credit: NASA

Could we peer even farther into the universe’s past?  Unfortunately, no.  The universe did not become transparent until it was 380,000 years old, when it cooled down sufficiently for light photons to pass unabated without colliding into densely packed particles.  It’s similar to seeing the surface of a cloud and not beyond.

The first nine minutes of the COBE probe produced a spectrum of the CMB.  The data was plotted against the predicted results of a blackbody spectrum.  The results are below:

Credit: Mather et al. Astrophys. J. Lett. , 354:L37ÐL40, May 1990

The data points are a perfect match for the prediction.  In fact, the CMB represents the most perfect blackbody spectrum observed.  The universe was in a hot, dense state after its creation.

The late 1990’s would add another twist to the expansion of the universe.  Two teams, one based in Berkeley and the other at Harvard, set out to measure the rate of expansion throughout the history of the universe.  It was expected that over time, the inward pull of gravity would slow the rate of expansion.  And this is what relativity would predict once the cosmological constant was pulled out, or technically speaking, equal to zero.  The two teams set about their task by measuring Type Ia supernovae.

Like Cepheid variables, Type Ia supernovae are standard candles.  They result when a white dwarf siphons off enough mass from a neighboring star to reach the size of 1.4 Suns.  Once this happens, a supernova occurs and as these transpire at the same mass point, their luminosity can be used to calibrate distance and in turn, the history of the universe.  A galaxy 1 billion light years away takes 1 billion years for its light to reach Earth.  A galaxy 8 billion light years away takes 8 billion years for its light to reach Earth.  Peering farther into the universe allows us to peer farther back in time as well.

What the two teams found sent shock waves throughout the world of astronomy.

The first 10 billion years of universe went as expected, gravity slowed the expansion.  After that, the expansion accelerated.  The unknown force pushing out the universe was dubbed dark energy.  This puts the cosmological constant back into play.  The key difference instead of operating from an incorrect assumption of a static universe, data can be used to find a correct value that would model an increasingly expanding universe.  Georges Lemaitre’s intuition on the cosmological constant had been correct.  While the exact nature of dark energy needs to be worked out, it does appear to be a property of space itself.  That is, the larger the universe is, the more dark energy exists to push it out at a faster rate.

Besides detecting the CMB, cosmologists spent the better part of the last century calculating the Hubble constant.  The value of this constant determines the rate of expansion and provides us with the age of the universe.  The original value derived by Hubble in the 1920’s gave an age for the universe that was younger than the age of the Earth.  Obviously, this did not make sense and was one reason scientists were slow to accept the Big Bang theory.  However, it was clear the universe was expanding and what was needed was better technology affording more precise observations.  By the time I was an undergrad in the 1980’s, the age of the universe was estimated between 10-20 billion years.

When the Hubble Space Telescope was launched in 1990, a set of key projects were earmarked for priority during the early years of the mission.  Appropriately enough, pinning down the Hubble constant was one of these projects.  With its high resolution able to measure the red shift of distant quasars and Cepheid variables, the Hubble was able to pin down the age of the universe at 13.7 billion years.  This result has been confirmed by the subsequent WMAP and Planck missions.

The story of the Big Bang is not complete.  There is the lithium problem. The Big Bang model predicts three times the amount of lithium as is observed in the universe.  And there is inflation.  In the first moments of the universe’s existence, the universe was small enough for quantum fluctuations to expand it greatly.  Multiple models exist explaining how this inflation occurred and this needs to be resolved.  This would determine how the universe is observed to be flat rather than curved and why one side of the universe is the same temperature as the other when they are too far apart to have been in contact.  An exponential expansion of the universe during its first moments of existence would solve that.

NASA’s WMAP mission measured the average angular distance between CMB fluctuations. One degree is flat, 0.5 degrees open, 1.5 degrees closed. The measurement came in flat at 1 degree consistent with inflation theory. Credit: NASA/WMAP

Then there is the theory of everything.  In his 1966 PhD thesis, Stephen Hawking demonstrated that if you reverse time in the Big Bang model, akin to running a movie projector in reverse, the universe is reduced to a singularity at the beginning.  A singularity has a radius of zero, an infinite gravity well and infinite density.  Once the universe has a radius less than 1.6 x 1035 meter, a quantum theory of gravity is required to describe the universe at this state as relativity does not work on this scale.

When discussing these problems with Big Bang skeptics, the tendency is to reply with a gotcha moment.  However, this is just scientists being honest about their models.  And if you think you have an alternative to the Big Bang, you’ll need to explain the CMB blackbody spectrum, which can only be produced by a universe in a hot dense state.  And you’ll need to explain the observed expansion of the universe.  It’s not enough to point out the issues with a model, you’ll need to replicate what it gets right.  While there are some kinks to work out, the Big Bang appears to be here to stay.

You don’t need access to an observatory or a NASA mission to experience the remnants of the Big Bang.  Every glass of water you drink includes hydrogen created during the Big Bang.  And if you tune your television to an empty channel, part of the static you see is noise from the CMB.  The Big Bang theory and the observational evidence that backs it up is one of the landmark scientific achievements of the 20th century, and should be acknowledged as such.

*Image atop post is a timeline of the evolution of the universe.  Credit:  NASA/WMAP Science Team.