Reading Practice 27

[1] In scientific history, there have been several attempts to explain the beginnings of our universe. However, it wasn’t until Einstein’s seminal publication on the theory of general relativity in 1915 that humans have been able to come up with an explanation that appears to hold true. Currently, the accepted model that explains the origins of our universe is called the Big Bang theory. According to this explanation, the universe was originally very compact, dense, and hot. However, ever since an enormous cosmic explosion – dubbed “the Big Bang” – the universe has been both expanding and cooling. While Einstein has earned a lot of fame and credit for his theories, there are other contributors without whom the Big Bang theory would not be as clearly articulated or widely accepted.

 

[2] The work that formed the basis of Einstein’s general theory of relativity was bolstered by the work of Russian scientist Alexander Friedmann. In 1922, this physicist provided solutions to the field equations that laid the foundation for subsequent work on the Big Bang theory. Further supporting evidence for Einstein’s work came in 1929, when Edwin Hubble discovered that the light of distant galaxies was shifted toward the red end of the light spectrum. This phenomenon, known as “redshift,” demonstrates that galaxies are, in fact, moving away from each other. Moreover, his discovery that more distant galaxies are moving at a faster rate demonstrates that the expansion of the universe is occurring in a uniform fashion. Hubble’s discoveries proved to be one of the most important keys in unlocking the mechanism of the Big Bang. But what exactly is this phenomenon known as redshift?

 

[3] Redshift can be understood through a more familiar analogous phenomenon. We have all had the experience of standing in the street and hearing the siren of an ambulance or police car approaching us. What happens when the moving vehicle passes us? The sound of that siren changes noticeably. We call this the “Doppler effect,” and it occurs because of the difference between sound waves of an object moving toward us and those of an object moving away from us. This explains the sudden change in the sound of the siren.

 

[4] With redshift, the same thing is true of light waves. When an object such as a star is in motion, the light that it radiates changes. More specifically, it moves toward the red end of the light spectrum. Just as sound waves are longer when we hear a vehicle moving away from us, red light waves are longer for an object moving away from us. Furthermore, the extent to which a star’s light shifts can provide information about how fast the object is moving. Thus, by measuring the redshift of distant galaxies, scientists are able to determine their velocity. This, in turn, provides important information about how the universe is changing. By understanding the velocity of moving galaxies, scientists can begin to piece together what the universe looked like in the past. And they can also make predictions about what the universe will look like in the future.

 

[5] If this all sounds too easy, it is. Just as with most other theoretical models and subjects of scientific study, there are confounding factors. One of these is that the light that reaches human observers from distant galaxies may appear red for reasons other than redshift. When light moves through dust clouds, the dust particles may scatter the incoming light. In some cases, the dust can scatter the blue wavelengths of light while allowing more red light through. The result, from a human perspective, is red light from a distant galaxy that is not caused by redshift. That is, the red that reaches our eye tells us nothing about that galaxy’s velocity. Besides this physical interference of dust, there is also the fact that scientists’ ideas of the past and future are still in the realm of theory. There is simply no way of confirming that our reconstructions of the universe in the past are accurate.