Assuming that dark energy is accelerating the expansion of the universe, how exactly is it doing it?
At the end of last month, the American government approved the SNAP project, a spacecraft that will be launched to study "dark energy", a factor with unknown properties that accelerates the expansion of the universe.
Indirect observational evidence for the existence of dark energy was discovered four years ago. It cannot be observed directly, but scientists believe that it fills the entire space and pushes the universe to expand at an increasing speed. The Probe (Supernova Acceleration SNAP) spacecraft will look for and photograph supernovae - stars that ended their lives in a massive explosion. The photographs and measurement of the light properties of many supernovae - which will be done using a telescope and a unique digital camera, thirty times larger than the Hubble Space Telescope's camera - will allow, so the scientists hope, a better understanding of the essence of dark energy.
In 1929, the American astronomer Edwin Hubble published the findings of a series of observations, which show that the different galaxies are moving away from each other at a high speed. The observation led to the formation of the "Big Bang" theory, which believed that the universe was initially concentrated in one point and then exploded and began to expand and grow. To visualize the picture of the world obtained from this theory, the image of the "inflating balloon" can be used. The different galaxies are dots drawn on the surface of a balloon. In the initial state, when the balloon is empty, the dots are very close to each other. With the swelling the points move away, each one from all the others.
Until the previous decade, the accepted assumption among physicists was that gravity would slow down the rate of expansion of the universe. If the amount of matter in the universe is large enough, gravity will stop the expansion of the universe completely. The universe will stop expanding and begin to collapse back under its own weight, perhaps to the point of re-concentration into a single point. This possibility was called the "hot end" - a concentrated and boiling universe (one can think of the possibility that a collapse of this type may lead to the creation of a new big bang and the possibility of the existence of an endless cycle of universes being created and dying). Alternatively, if the amount of matter in the universe is not sufficient, it is expected to die a "cold" death - it will continue to expand forever, the galaxies will move away from each other, the universe will lose its vitality, stop radiating light and disperse into its basic elements.
In the mid-XNUMXs, with the maturation of appropriate technologies and observation tools, two groups of scientists set out to measure the slowing rate of the universe's expansion. The two groups - an American group led by Dr. Saul Perlmutter from the University of California at Berkeley and an Australian group led by Brian Schmitt - examined the light coming from supernova explosions for the purpose of the research. Supernovae serve as valuable research tools because they are very bright events that occur at vast distances. The researchers measured the brightness of each supernova and its "redshift" - the change in the frequency of the light resulting from the fact that it is moving away from our observation instruments.
When the light that reaches us from bodies in space is separated into the different colors that make it up, the "light spectrum" pattern is obtained. By investigating the spectrum it is possible to learn about the composition and movement of the element emitting the light. As the source moves away from the Earth, a shift of the spectrum pattern can be observed towards the low-frequency red color. This is similar to the Doppler effect, which causes us to hear a higher sound from a train when it is approaching us and a lower sound when it is moving away.
By measuring the brightness of supernovae and their redshift, it was possible for the first time to obtain data about the rate of expansion of the universe during different periods of its life. The results of the two studies, published in 1998, stunned the scientific world. Instead of accepting one value or another for the slowing down of the expansion of the universe, the two groups discovered at the same time that the universe is not slowing down at all, but on the contrary - it is accelerating its expansion. The immediate question that arose was what causes this acceleration.
The researchers hypothesized that there is probably a factor, the nature of which is unknown, which is everywhere in the universe and accelerates its spread. The term claimed for this factor, "dark energy", emphasizes the uncertainty that physicists feel regarding it. The scientific hypothesis accepted as an explanation for the phenomenon is that dark energy is a product of empty space itself.
Amazingly, the idea of energy emanating from space itself was predicted by Albert Einstein in his theory of general relativity. Einstein assumed that there should be energy emanating from space itself based on the development of the equations of his theory. The accepted opinion at that time was that the universe is static and does not expand or contract. Since the general theory of relativity in its initial form does not allow the universe to remain in a stable state, Einstein was forced to add another element to the equations. He called this element the "cosmological constant".
At the end of the twenties, it became clear, as mentioned, that the universe is expanding and the Big Bang theory was proposed, which explains its expansion. In view of these changes, Einstein retracted his statement, canceled the cosmological constant and said that adding it to the equations was "the biggest mistake of my life". However, in light of the new measurements that revealed that the expansion of the universe is accelerating and in light of further developments in physics, it became clear to scientists that given an appropriate value, Einstein's cosmological constant can accurately predict the acceleration in expansion and explain dark energy. It is therefore possible that Einstein was right in his initial theoretical thought.
It should be noted that according to another modern physical theory, quantum mechanics, there is a good basis for the assumption that empty space does produce energy and can be a source of dark energy. The problem is that the predictions of the intensity of the energy predicted by quantum mechanics are much greater than the intensity measured by the astronomers - 10 times to the 50th power to 10 times to the 123th power.
What are the properties of dark energy? Different physical models provide different answers to this question. According to Einstein's model, the energy contained in a certain volume of space remains constant throughout the evolution of the universe. According to another model, dark energy varies in space and time. Dr. Perlmutter and another researcher from the University of California at Berkeley, Michael Levy, came up with the idea of launching a spacecraft that would investigate more precisely many supernova events in order to try and find out what the correct physical explanation for dark energy is. The "Snap" spacecraft is a product of this idea.
During its mission, which will last three years, Snap will image thousands of supernovae and also measure their redshift using a spectrograph instrument. From its orbit around the Earth, the spacecraft will be able to provide more and more accurate information than can be obtained from ground observations, and according to this information the researchers will try to develop a model that describes the dark energy. According to Greg Aldring, from the SNAP research team at the Lawrence Laboratory in Berkeley, "When we see how dark energy changes in space and time, we hope to be able to choose between the different models that explain it."
Snap's sensor system will contain a telescope that includes a mirror with a diameter of two meters and a camera with 600 million elements (pixels). For comparison, the camera of the Hubble Space Telescope launched in 1990 contains only two million pixels. The cost of the project is estimated at several hundred million dollars. Intended launch year: 2008.
