The Hubble Law
In 1926, astronomer Edwin Hubble, while investigating the spectrum of some faint distant galaxies, discovered an unusual relationship. The spectrum of each of the galaxies he investigated was shifted toward the red end of the spectrum (called redshift). Even more interestingly, the fainter the galaxy, the larger the redshift. Hubble reasoned that the fainter a galaxy was, the more distant it was. But if that were true, then the more distant a galaxy, the larger its redshift. This distance-redshift relationship eventually became known as Hubble 's law.
Astronomers sought an explanation for this strange relationship. There were three possibilities that were believed to cause a redshift in light from a distant object. These were:
Einstein 's effect is called the Gravitational Redshift, and it has been experimentally verified here on Earth (the Mössbauer effect), the Vessot rocket clock experiment, for light from the Sun, and for several other objects in the heavens. The problem is that it is far too small to account for the redshifts measured from distant galaxies, and so is not thought to be the cause of the redshift. In addition, this effect is strictly a function of the mass and diameter of an object, and would not vary with distance.
A similar effect happens with light. If an object such as a star is approaching Earth, the frequency of the light received from the star increases slightly, shifting slightly toward the blue end of the spectrum. This is called a blue shift, and has been observed in a large number of nearby stars and some nearby galaxies.
Conversely, if the star is moving away from Earth, the frequency of light received from the star decreases, and the light is shifted slightly toward the red end of the spectrum. This is called a redshift, and is also observed in many nearby stars.
Since redshifts and blueshifts had already been observed in nearby stars, astronomers were predisposed to accept the Doppler effect as the cause of the redshift of distant galaxies, as discovered by Hubble. Thus, in the early part of this century, it was decided that the Doppler effect was the cause of Hubble's redshift, and that all distant galaxies were receding from us. This explanation, which was really just an assumption because no other explanation could be found, has been the foundation of astronomy for the last 70 years.
The Expanding Universe Theory
With the Doppler effect established as the accepted explanation for the redshift of distant galaxies, there came a major problem. All distant galaxies showed only a redshift, which meant that all galaxies were receding from us. But what Hubble 's law showed was that the further away they were, the higher the redshift. And in all directions! More distant galaxies appeared to be moving away at much larger velocities than those closer to us. The universe appeared to be expanding like some gigantic balloon being blown up, or some cosmic explosion.
If the distance-redshift relationship for faint galaxies is due to the Doppler effect, as believed by most astronomers, then the universe must be expanding to provide the effects observed. But remember, the Doppler effect has not been proven to be the cause of the Hubble effect. It is merely an assumption by astronomers, selected as the best alternative seventy years ago. If another cause of the distance-redshift relationship were found, the universe might not be expanding at all!
The Big Bang
The belief that the universe is expanding is almost universal among astronomers. Gradually the concept developed that the expanding universe was similar to the expanding fireball of an explosion. It appeared that the universe we now see could be the remnants of a colossal cataclysm in the distant past. That is, astronomers came to believe that the universe was created by a gigantic explosion billions of years ago. This concept was dubbed the Big Bang!
Like running a movie in reverse, astronomers decided they could learn about the creation of the universe by running the expansion of the universe backward until they discovered how and when the universe started. Thousands of astronomers have spent most of their career studying the Big Bang. Some are even experts in the tiniest fraction of a second before the Big Bang!
But there are serious problems with the accepted standard Big Bang theory. As the theory has evolved over the decades, many discrepancies between the theory and observations have appeared, each requiring modifications and "tweaking" of the theory. Today it is like a house of cards, with very tight limits on a large number of empirical parameters which have had to be added to make the theory fit observations. Worse yet, there is a growing discrepancy between the age of the universe, as provided by the Big Bang theory, and observations of the age of distant galaxies, and even some nearby stars.
Remember that the Big Bang theory is a direct result of the belief that the universe is expanding, which in turn is based on the assumption that the Doppler effect is the cause of the distance-redshift relationship for distant galaxies. If another explanation for this relationship is found which does not require an expanding universe, then there is no reason to believe there ever was a Big Bang. And there is another explanation -- one that has been experimentally proven!
The Shapiro Effect
You may not have heard of the Shapiro effect before, but you are about to find out that it is the explanation for the distance-redshift effect discovered by Edwin Hubble, and it has been proven experimentally many times!
It all started with a short letter in the Journal of Astrophysics in 1964 by Dr. Irwin I. Shapiro of the Lincoln Labs of the Massachusetts Institute of Technology, which stated in part:
"...according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path."
He was speaking, of course, of Einstein 's General Theory of Relativity. Dr. Shapiro, it seems, was the first to make use of a previously forgotten facet of relativity theory — that the speed of light is reduced when it passes through a gravitational field.
In this landmark letter, Dr. Shapiro went on to suggest that this new test of relativity theory could be verified by observing the time delay of radar signals returned from the surface of the planets Venus and Mercury. He estimated that the effect of the sun 's gravitational field on the radar beam would be to cause a delay of as much as two hundred microseconds (0.0002 seconds) in the round trip travel time of a radar signal returned from a distant planet. The maximum delay would occur at the beam 's closest approach to the sun. He went on to explain how, with the knowledge and technology available (in the mid 1960 's), such a test could be successfully made to within five to ten percent accuracy using the MIT Haystack radar.
His idea was to bounce radar beams off the surface of the planets Venus and Mercury, and measure the total time it took for the beams to go from the earth to these planets and return. Since the relative positions of the planets and earth are known quite accurately, the expected travel time of the radar beam could be computed with great accuracy as well. His solution of Einstein 's equations of relativity indicated that as the radar beam passed closer and closer to the sun, there would be a small time delay. The total time for the radar beam to go from the earth to the planets and back, at the closest approach of the radar beam to the sun, would be increased by 200 microseconds compared to what would be expected if the sun were not there. This is a relatively easy time difference to measure.
Dr. Shapiro was right! The first test of this new aspect of Einstein 's theory was resoundingly successful, matching not only the predicted amount of time delay, but also the relationship predicted by Einstein as well. That relationship is very important, as we shall soon see.
Further Tests of the Shapiro Effect
The first experiments with the MIT Haystack radar and the distant planets were highly successful, but relatively crude by modern standards. The experiments have been repeated many times since, with increasing accuracy, until today the deviation between the experimental results and solutions to Einstein 's equations is less than one percent. Thus the measurement of the predicted time delay is one more verification that Einstein 's general theory of relativity is correct.
One of the modern set of experiments measured time differences from signals returned by transponders on the Mariner 6 and Mariner 7 spacecraft as they orbited the planet Mars. This a far more accurate test than simply bouncing radar beams off the surface of a planet, since surface irregularities introduce an element of error which cannot be controlled. The use of highly accurate and controlled transponders aboard these satellites significantly reduced the errors present. These experiments, coupled with numerous data collected during the Mariner program, led to refinements in many of the variables involved in the test, such as planetary motion, solar corona effects, etc., further reducing the potential error sources.
Perhaps the most accurate experiments of the Shapiro effect have been conducted as a result of NASA 's Viking project. This program placed unmanned landing craft on the surface of the planet Mars to explore its characteristics. One of the wonderful results of this program, you may recall, was to return colour photographs of the Martian surface. A lesser known part of this program was to leave transponders on the surface of Mars. These transponders respond to radio signals from earth and return, or "echo" these signals back to earth, ideal for testing the gravitational time delay. Such controlled signal response from fixed positions eliminates both the random nature of raw radar returns from a planetary surface, and possible orbital variations present when returning signals from the Mariner spacecraft.
As the line of sight between Earth and Mars drew closer and closer to the sun, a measurable excess time delay began to occur. When the line of sight came nearest to the Sun (called superior conjunction), the maximum excess time delay occurred — about 200 microseconds as predicted by Shapiro 's equations.
Dr. Shapiro 's discovery, which has now been named The Shapiro effect or gravitational time dilation, is extremely important from two aspects — it proves that light rays lose velocity (and thus energy) when passing through a gravitational field, resulting in a redshift, and that the effect is a long-range one!
A Long-Range Effect
Let me explain the importance of this long-range aspect. The bending of light by the sun or any other massive object, as well as the attractive force of gravity, are short-range effects — They die off very quickly with distance. The deflection of light just grazing the surface of the sun is 1.75" (arc-seconds), or about 1 / 2000th of a degree. At one hundred times the sun 's radius the effect has dropped to one percent of this value. This is a short-range effect.
The Shapiro effect, on the other hand, is a long-range effect. Instead of the time delay effect decreasing with the inverse of the distance from the center of the sun ( 1/d ), as does the bending of light, the Shapiro effect decreases with the inverse of the logarithm of distance ( 1 / ln (d) ). If the time delay at closest approach to the Sun is 200 microseconds, at one hundred times the sun 's radius the effect has dropped to 44 microseconds, or twenty two percent of its maximum. In contrast, at this distance the bending of light has dropped to one percent of its value at the surface of the sun. The time delay is still 14% of its maximum value when the radio beam passes at 1,000 times the solar radius. At this distance the bending of light is insignificant. Thus the Shapiro effect is a long-range effect! The importance of this logarithmic aspect of a gravitational effect is major, and has been totally ignored.
The Shapiro Effect and Redshift of Distant Galaxies
So what does this mean? Imagine light being emitted from a distant galaxy one hundred million light years away (meaning the light has to travel a hundred million years to reach us). As that light wends its way tirelessly toward earth, it passes continually through the extremely small but ever-present gravitational field present in outer space — the cumulative gravitational field of every star and galaxy along its path. As we now know from Dr. Shapiro 's experiments, that light will experience a small, cumulative deceleration from the forces of this gravitational field acting at long range. This is not an interaction of the light photon with interstellar matter, which would smear the light and provide a visible signature, but simply a gravitationally induced time delay. The photon 's interaction with gravity does not alter its path, nor change its characteristic, except by gradually decreasing its velocity and energy.
By the time light from a distant galaxy reaches Earth and our telescopes, it has less velocity than when it left its host star, and thus less energy than when it started. In a word, it is redshifted! And not because the distant source galaxy is receding, but because the inter-galactic gravitational field has reduced its energy. The Shapiro effect has shown us that a redshift is to be expected, which increases with distance! That is just what Hubble observed, but with an explanation which does not require the Doppler effect, an expanding universe, or a Big Bang.
Lot 's of Gravity Out There!
It may seem that there is not enough gravitational force out in distant space, far away from celestial objects, to cause the redshift in light to equal what has been measured for the distant galaxies. But that is not true. For many years astronomers have studied the motion of galaxies as their paths through the cosmos are affected by the gravitational attraction of other galaxies millions of light years away. Recent results from the Infrared Astronomical Satellite (IRAS) and Cosmic Background Explorer (COBE) satellites investigated very small "dipole" variations in their measurements of background energy in the universe surrounding us. These variations take the form of a very slight increase in the temperature of the background radiation in the direction the earth is traveling, and a corresponding decrease in energy in the direction opposite the earth 's motion. These small dipole measurements then can be used to determine the velocity of the earth in orbit around the sun (which, of course, is very well known), the direction and velocity of motion of our solar system relative to our local galaxy, and something which could not have been accurately measured before, the motion of our local galaxy through space relative to other distant galaxies.
The results of these investigations have determined that our galaxy is being pulled generally in the direction of the galaxies Hydra, Centaurus and Virgo by their combined gravitational attraction, and is now streaming toward these galaxies at more than a million miles per hour due to the cumulative effect of their gravitational fields. Since these galaxies are as much as one hundred million light years away, they must exert a tremendous gravitational pull to drag an entire galaxy such as ours.
Gravitational forces in the universe most certainly cannot be considered negligible! And remember that the gravitational attraction of these distant galaxies is the short-range force that drops off with the square of the distance. The gravitational force that causes the Shapiro effect is of a different kind. It is a long-range force, since it decreases only with the inverse log of the distance. Because of this long-range nature, there should be ample opportunity for light to be gravitationally slowed as it passes hundreds of millions of light years through space to our telescopes. Because the effect is cumulative, even the smallest reduction in the velocity of a photon over the period of a year due to gravity is multiplied by a million times or more in its travel towards earth. And this is where the redshift of light from distant galaxies comes from, not the Doppler effect!
The Shapiro Effect and Hubble 's Law
We have seen that the Shapiro effect, or the gravitational time delay, as predicted by Einstein, should cause light from distant galaxies to lose energy and thus be redshifted. And since the amount of redshift would be dependent on distance, the redshift should be correlated with distance. In other words, Hubble 's law, as it relates to galaxies, is correct. However, there is a very important point to be made here. What we have said does not change the fundamental Hubble law in its original form, that redshift is a measure of distance for galaxies (but not quasars). But what the Shapiro effect shows is that the redshift from distant galaxies is not due to the Doppler effect and velocities of recession, but a natural result of the influence of the inter-galactic gravitational field on the propagation of light.
We cannot expect the Shapiro effect to account for the very large redshifts found in some objects such as quasars. That would require intergalactic gravitational fields far larger than one could reasonably expect. The redshift caused by the Shapiro effect applies only to the relatively small redshifts from galaxies, those first studied by Hubble in formulating his Hubble law. The cause for the much higher redshifts seen in quasars will be addressed separately.
A Second Factor — Gravitational Waves
A second factor which has not been considered in the loss of energy of a light beam passing through space is gravitational waves. Einstein showed that an object experiencing acceleration due to gravitational forces emits gravitational waves, thereby losing energy. This was the basis for his famous explanation for the discrepancy in the orbit of the planet Mercury around the sun. Recent experiments on a binary pulsar system by Hulse and Taylor have verified Einstein 's predictions to within one percent. What we tend to forget is that a photon of light passing through deep space also experiences acceleration forces due to gravity, and thus would emit gravitational waves. This emission would then lower the energy of the photon, so that even if it reached us with its initial velocity intact, it would have lower energy than when it was first emitted, and thus be redshifted.
Proof that Gravity reduces the velocity of light!
The Pioneer 10 and Pioneer 11 spacecraft were launched into space in January, 1987, and were the first spacecraft to exit the solar system. Their trajectories were followed for many years with extreme precision until they were too weak to respond. Detailed analysis showed a strange anomaly in their positions. It appears they were drawn slowly toward the Sun beyond anything predicted by the Theory of Gravity.
A detailed 55 page analysis was conducted to attempt to explain this anomaly, but failed to do so. The only plausible explanation is that the velocity of light in the vicinity of the Sun is slightly less than expected. This is in complete agreement with what would be expected from Gravity Drag (i.e. the Shapiro effect)!
The Bottom Line
Quite simply, the Hubble law, which relates the redshift of distant galaxies to their distance, may be not due to the Doppler effect, as is universally accepted in the astronomical community, but could be due to the effect of intergalactic gravitational fields on light. This means that there is no reason to believe that the universe is expanding, and therefore no reason to believe there was ever a Big Bang.