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In other words, the stars themselves would have burned out long ago, dissipating their heat into surrounding space.
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The fact that there are still active stars must mean that the universe has existed for a finite amount of time, and was created at some specific point in time. Perhaps the age of that point in time could be determined? At about the same time, an Austrian physicist by the name of Christian Doppler was studying astronomy and mathematics.
Doppler knew that light behaved like a wave, and so began to think about how the movement of stars might affect the light emitted from those stars. In a paper published in , Doppler proposed that the observed frequency of a wave would depend on the relative speed of the wave's source in relation to the observer, a phenomenon he called a "frequency shift" Doppler, He made an analogy to a ship at sail on the ocean, describing how the ship would encounter waves on the surface of the water at a faster rate and thus higher frequency if it were sailing into the waves than if it were traveling in the same direction as the waves.
You might be familiar with the frequency shift, which we now call the Doppler Effect in his honor, if you have ever listened to the sound of traffic while standing on the side of the road. The familiar high-to-low pitch change is an example of the effect — the actual frequency of the waves emitted is not changing, but the speed of the passing vehicle affects how quickly those waves reach you. Doppler proposed that we would see the same effect on any stars that were moving: Their color would shift towards the red end of the spectrum if they were moving away from Earth called a redshift and towards the blue end of the spectrum if they were moving closer called a blueshift see Figure 4.
He expected to be able to see this shift in binary stars , or pairs of stars that orbit around each other. Eventually, Doppler's paper, entitled "On the coloured light of the double stars and certain other stars of the heavens," would change the very way we look at the universe. However, at the time, telescopes were not sensitive enough to confirm the shift he proposed.
Doppler's ideas became part of the scientific literature and by that means became known to other scientists. By the early s, technology finally caught up with Doppler and more powerful telescopes could be used to test his ideas.
In September of , an American named Vesto Slipher had just completed his undergraduate degree in mechanics and astronomy at Indiana University. He got a job as a temporary assistant at the Lowell Observatory in Flagstaff, Arizona, while continuing his graduate work at Indiana. Shortly after his arrival, the observatory obtained a three-prism spectrograph , and Slipher's job was to mount it to the inch telescope at the observatory and learn to use it to study the rotation of the planets in the solar system.
After a few months of problems and trouble-shooting, Slipher was able to take spectrograms of Mars, Jupiter, and Saturn. But Slipher's personal research interests were much farther away than the planets of the solar system. Like Doppler, he was interested in studying the spectra of binary stars , and he began to do so in his spare time at the observatory. Over the next decade, Slipher completed a Master's degree and a PhD at Indiana University, while continuing his work at Lowell Observatory measuring the spectra and Doppler shift of stars. In particular, Slipher focused his attention on stars within spiral nebulae Figure 5 , expecting to find that the shift seen in the spectra of the stars would indicate that the galaxies those stars belonged to were rotating.
Indeed, he is credited with determining that galaxies rotate, and was able to determine the velocities at which they rotate. But in , having studied 15 different nebulae, he announced a curious discovery at a meeting of the American Astronomical Society in August:. In the great majority of cases the nebulae are receding; the largest velocities are all positive The striking preponderance of the positive sign indicates a general fleeing from us or the Milky Way. Slipher had found that most galaxies showed a redshift in their spectrum , indicating that they were all moving away from us in space, or receding Slipher, By measuring the magnitude of the redshift, he was able to determine the recessional velocity or the speed at which objects were "fleeing.
Slipher continued his work with redshift and galaxies and published another paper in , having now examined 25 nebulae and seen a redshift in 21 of them. He extended Slipher's measurements to the entire universe , and calculated mathematically that the universe must be expanding in order to explain Slipher's observation. Einstein's criticism had a personal and cultural component, two things we often overlook in terms of their influence on science. Several years earlier, Einstein had published his general theory of relativity Einstein, In formulating the theory, Einstein had encountered one significant problem: General relativity predicted that the universe had to be either contracting or expanding — it did not allow for a static universe.
But a contracting or expanding universe could not be eternal, while a static, non-moving universe could, and the prevailing cultural belief at the time was that the universe was eternal.
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Einstein was strongly influenced by his cultural surroundings. As a result, he invented a "fudge factor," which he called the cosmological constant , that would allow the theory of general relativity to be consistent with a static universe. But science is not a democracy or plutocracy; it is neither the most common or most popular conclusion that becomes accepted, but rather the conclusion that stands up to the test of evidence over time. Einstein's cosmological constant was being challenged by new evidence. Scientists are not influenced by their personal experiences, their beliefs, or the culture of which they are a part.
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In , an American astronomer working at the Mt. Wilson Observatory in southern California made an important contribution to the discussion of the nature of the universe. Edwin Hubble had been at Mt. Wilson for 10 years, measuring the distances to galaxies, among other things. In the s, he was working with Milton Humason, a high school dropout and assistant at the observatory.
Hubble and Humason plotted the distances they had calculated for 46 different galaxies against Slipher's recession velocity and found a linear relationship see Figure 6 Hubble, In other words, their graph showed that more distant galaxies were receding faster than closer ones, confirming the idea that the universe was indeed expanding. This relationship, now referred to as Hubble's Law , allowed them to calculate the rate of expansion as a function of distance from the slope of the line in the graph.
This rate term is now referred to as the Hubble constant. Knowing the rate at which the universe is expanding, one can calculate the age of the universe by in essence "tracing back" the most distant objects in the universe to their point of origin. Using his initial value for the expansion rate and the measured distance of the galaxies, Hubble and Humason calculated the age of the universe to be approximately 2 billion years. Unfortunately, the calculation was inconsistent with lines of evidence from other investigations. By the time Hubble made his discovery, geologists had used radioactive dating techniques to calculate the age of Earth at about 3 billion years Rutherford, — or older than the universe itself!
Hubble had followed the process of science, so what was the problem? Even laws and constants are subject to revision in science. It soon became clear that there was a problem in the way that Hubble had calculated his constant. In the s, a German astronomer named Walter Baade took advantage of the blackouts that were ordered in response to potential attacks during World War II and used the Mt. Wilson Observatory in Arizona to look at several objects that Hubble had interpreted as single stars.
With darker surrounding skies, Baade realized that these objects were, in fact, groups of stars, and each was fainter, and thus more distant, than Hubble had calculated. Baade doubled the distance to these objects, and in turn halved the Hubble constant and doubled the age of the universe. In , the American astronomer Allan Sandage, who had studied under Baade, looked in more detail at the brightness of stars and how that varied with distance. The new estimates developed by Baade and Sandage did not negate what Hubble had done it is still called the Hubble constant , after all , but they revised it based on new knowledge.
The lasting knowledge of science is rarely the work of an individual, as building on the work of others is a critical component of the process of science. Hubble's findings would have been limited to some interesting data on the distance to various stars had it not also built on, and incorporated, the work of Slipher. Similarly, Baade and Sandage's contribution were no less significant because they "simply" refined Hubble's earlier work.
Since the s, other means of calculating the age of the universe have been developed. For example, there are now methods for dating the age of the stars, and the oldest stars date to approximately The Wilkinson Microwave Anisotropy Probe is collecting data on cosmic microwave background radiation Figure 7. Using these data in conjunction with Einstein's theory of general relativity , scientists have calculated the age of the universe at The convergence of multiple lines of evidence on a single explanation is what creates the solid foundation of scientific knowledge.
Why should we believe what scientists say about the age of the universe? We have no written records of its creation, and no one has been able to "step outside" of the system , as astronauts did when they took pictures of Earth from space, to measure its age. Yet the nature of the scientific process allows us to accurately state the age of the observable universe.
These predictions were developed by multiple researchers and tested through multiple research methods. They have been presented to the scientific community through publications and public presentations. And they have been confirmed and verified by many different studies. New studies, or new research methods, may be developed that might possibly cause us to refine our estimate of the age of the universe upward or downward.
This is how the process of science works; it is subject to change as more information and new technologies become available. But it is not tenuous — our age estimate may be refined, but the idea of an expanding universe is unlikely to be overturned.
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As evidence builds to support an idea, our confidence in that idea builds. Upon seeing Hubble's work, even Albert Einstein changed his opinion of a static universe and called his insertion of the cosmological constant the "biggest blunder" of his professional career. Hubble's discovery actually confirmed Einstein's theory of general relativity , which predicts that the universe must be expanding or contracting. Einstein refused to accept this idea because of his cultural biases.
His work had not predicted a static universe, but he assumed this must be the case given what he had grown up believing. When confronted with the data , he recognized that his earlier beliefs were flawed, and came to accept the findings of the science behind the idea.
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