From my previous blog on the legacy of Albert Einstein, he formulated the General Theory of Relativity in 1915 and became a celebrity in the world of science. General Relativity says that mass affects the shape of space and the flow of time. Gravity is the manifestation of space warped by mass, the greater the mass, the greater the warp. Einstein, like most of the scientists of his time, did not realize that the universe was expanding. According to the solutions from the field equations, it did suggest that the universe was either expanding or contracting. But Einstein was determined that the universe was static, ( neither expanding nor contracting) and he added a hypothetical term which is a repulsive force that would balance out the gravitational pull and prevented the universe from collapsing. He called the term ” cosmological constant “.
Then in the 1920s, astronomer Edwin Hubble who was using a type of star called a Cepheid variable as a ” standard candle” to measure distances to other galaxies, discovered that the universe was expanding. This idea of an expanding universe revolutionized astronomy. For the very first time, astronomers realized that Milky Way isn’t the entire universe, but there are many other galaxies in the universe much like our own, and they were receding away at very high velocities. The greater the distance away, the greater the speed of recession, hence the Hubble’s Law was named. If the universe was expanding, it must at one time have been much smaller, and this concept led to the “Big Bang Theory”, that the universe began as a tiny point that suddenly expanded to create everything we know today. The Hubble telescope built in the late 1980s has captured about 100 billion galaxies, each contain about 100 billion stars within the visible universe. Edwin Hubble sent an invitation to Einstein and had him looked through the 100 inch Hooker telescope at Mount Wilson, ( the largest telescope at that time). When Einstein was convinced that the universe was expanding, he quickly discarded the cosmological constant and called it ” the biggest blunder of his life”. But was it really a blunder?
Supernovae, which occur within a galaxy about every 100 years, are among the brightest events in the sky. When a star explodes, it releases a tremendous amount of energy that it can outshine all the stars in the galaxy. Astronomers use a certain type of exploding star called Type 1a supernova which occur in a binary system—two stars orbiting one another in which one of the stars has to be a white dwarf. White dwarf stars are one of the densest forms of matter next to neutron stars and black holes ( a teaspoonful of matter would weigh about 5 tons), their gravity is particularly intense. When it reaches 1.4 solar masses, or roughly 40% more massive than the sun, it will explode and the resulting light is around 5 billion times brighter than the sun, hence Type 1 a supernova. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the astronomer who discovered it. To find the distance to the galaxy that contains the supernova, astronomers compare the absolute brightness of the explosion to the apparent brightness and applying the inverse square law, they can calculate the distance to the supernova and to the supernova’s home galaxy.
In 1998, a team of 3 scientists, Saul Perlmutter of Lawrence Berkeley Lab, Adam Riess of Baltimore’s Space Telescope Science Institute and Brian Schmidt of the Australian National University, discovered that the universe expansion was accelerating using the data from Type 1a supernovae. For the next 10 years gathering substantial evidence to conclude the universe expansion is indeed accelerating, they shared the Nobel Prize in the year 2011. Scientists attribute to a mysterious entity dubbed “dark energy” , but what exactly is it? The best answer I can give you is that we don’t know. It does seem to contradict many of our understandings about the way the universe operates. Unlike dark matter which seems to clump things together, dark energy tends to push things apart acting opposite to the force of gravity. A kind of anti-gravitational action tearing the universe apart, a form of negative pressure being exerted on the universe. To make it clear about the meaning of expansion, it isn’t the galaxies that are expanding, you and I are certainly not expanding ( unless if we consume KFC and/or Winchell’s donuts on a daily basis), that would be quite a different story. What really expands is the space in between the galaxies. Picture a balloon with dots drawn on its surface and as we inflate the balloon, the space in between the dots will expand while the dots remain exactly the same size and this is what’s going on in the expansion of the universe. The more the universe expands, the more dark energy there seems to be, and therefore the more it drives the expansion. This special character indicates dark energy is not created by any known particles. Its influence increases as the universe ages while the effects of matters dissipate. If this trend continues, the universe will keep on expanding and eventually the galaxies will be so far apart from each other that we no longer will see them and in the far future, they will become invisible from one another. This would bring a halt to all astronomy and the end of the universe where everything will be completely torn apart. ( Big Rip).
So dark energy is something that is very strange but its effect is definitely observable. It pushes all the galaxies apart from each other as if it is resisting gravity. The most popular candidate is vacuum energy, the energy from quantum fluctuations in empty space. This effect of the vacuum energy can be recognized in Einstein’s theory of General Relativity if you recall the term that he discarded more than 80 years ago from the field equation and calling it the biggest blunder in his life, “cosmological constant”. The term was created in the equation to fight gravity so that the universe would not collapse even though the solution to the equation suggested that the universe should be expanding. If Einstein was willing to accept what the solution suggested instead of clinging to the static universe, he could have won another Nobel Prize along with Edwin Hubble for the second time, making him the only scientist ever won two Nobel Prizes in history. From General Relativity, any energy has a gravitational effect, and dark energy would be the one which accelerates the expansion of the universe. How profound this could have been? Many other observations have been made and shown that the effect of dark energy has not changed over time which rules out another popular model “quintessence”, a hypothetical fluid with odd properties that is dynamic, changing its effects over time of the cosmos. ” Cosmological constant” remains as the top candidate for dark energy based upon observations of many known supernovas.
But there are theories which oppose the idea of dark energy. This isn’t too surprising to me as General Relativity has not been tested on the cosmological scale despite the fact that it is the best theory of gravity tested in the solar system. It is possible the existence of dark energy is a sign of failure of general relativity on the largest scale and needs to be modified. Several modified theories of gravity have drawn inspiration from quantum physics which I will not discuss here, but perhaps on my next blog. These modified theories utilize a very different set of equations than the general relativity and were able to reproduce the known behavior of gravity in the solar system but act differently on much larger scales. Do we have enough evidence to suggest modification of gravity? Perhaps even modify general relativity to accommodate quantum mechanics? The truth about science is: no matter how elegant a theory seems to be, it must be able to fit the data. As more data come in, the theory must match all the observational evidence or it will be discarded or modified to supersede the previous one. This is falsification which is the most unique and efficient trait of all science.
There are many ongoing experiments globally searching for direct evidence of dark energy and its identifications. These projects all have similar goals but can be broken into three main categories. 1) Plot the expansion rate of the universe by measuring supernovae at different distances from Earth to see if it has changed over time which then reveal the influence of dark energy. 2) Measure the distances between galaxies at different times in the history of the universe. 3) Look for ” clumpiness” in the universe by studying the shapes of the galaxies.
Here are some of the more significant dark energy experiments:
BOSS: Baryon Oscillation Spectroscopic Survey, Sloan Digital Sky Survey telescope at New Mexico.
DES: Dark Energy Survey. 570-megapixel camera in South America
WFMOS: Wide-Field Multi-Object Spectrograph. Measures imprint of sound waves. Located in Hawaii.
LSST: Large Synoptic Survey Telescope. Scheduled for 2020. To plot the distribution of galaxies.
WFIRST: Wide-Field Infrared Survey Telescope. Study the universe at infrared wavelengths after 2020.
Euclid: Probe the distribution of dark matter and measure distances to supernovae.
There is no doubt that searching for dark energy will remain as one of the hottest topics in cosmology research in the many years to come. Dark energy both signals that we will have a great deal to learn, beckoning the way toward an unexplored realm of physics and that we stand poised for another great leap in our understanding of the universe.