Saturday, June 16, 2018

Even Darker

In 1929 Edwin Hubble made a discovery that revolutionized our understanding of the universe. He measured the relative velocity between our Milky Way galaxy and 46 other galaxies and observed that the galaxies were all moving away from each other. In addition, the farther apart the galaxies were from each other, the faster they were receding from each other. The most straightforward interpretation of this observation is that the universe is expanding. Subsequent observations have revealed that the galaxies are not simply moving away from each other, but that the fabric of space itself between the galaxies is actually expanding.

With the knowledge that the universe is expanding, scientists have wondered about the future of the universe. Will it expand forever? Will it eventually stop expanding and collapse in on itself? The answer to these questions depends partially on the amount of matter in the universe because every object in the universe is attracted to every other object by the force of gravity. Gravity should cause the expansion of the universe to gradually slow down and, if there is enough matter, to eventually stop and collapse. To illustrate this consider two plastic balls connected by a rubber band. If you were to hold one ball in your hand and throw the other ball outward, eventually the speed of the second ball would decrease as the rubber band stretched because the rubber band acts as an attractive force between the two balls. In the same way, the attractive force of gravity should be causing the expansion of the universe to slow down. Because the strength of gravity depends on how much matter is in the universe, if there is enough matter, the very strong attractive force should eventually cause the expansion to stop and the universe to collapse back in on itself. The attractive force of gravity should be slowing down the expansion of the universe. But that is not what is happening.

In 1998 and 1999 observations of distant supernova in the universe gave strong evidence that the rate of expansion of the universe was actually increasing. (Supernova are stars that have ended their lifetime in a spectacular explosion that is very bright and very noticeable.) This observation is exactly the opposite of what would be expected if gravity alone is acting on the objects.  The rate at which the supernova are separating is actually increasing and not getting slower. Although we can observe this acceleration we have no idea what is causing it. We call the cause of this effect "dark energy" where dark means that the cause is unknown, and energy means it is some kind of energy that is pushing things apart faster. Sometimes dark energy is described as being a force like "anti-gravity" since it is causing the expansion of the universe to speed up. But it is not really anti-gravity, for it is not causing objects with mass to accelerate away from each other faster, but causing the physical space between the objects to stretch and expand faster and faster.

Let me get technical for a moment and describe one of the measurements that indicates the rate of expansion of the universe is increasing. (If you don't care about these technical details you can skip this paragraph and not miss anything.) One method for measuring the expansion rate is quite simple in principle. If you know how far away an object is and how fast the object is receding, then you can compare the distance and speed with what would be expected if the expansion rate of the universe is slowing down or speeding up. So how do we know how far away an object is and how fast it is moving away from us? In the supernova measurement, type IA supernova are used as a "standard candle" to determine their distance from us. To illustrate a standard candle, consider an object that you know exactly how bright it is, like maybe a 100 watt light bulb. If you are close to the light bulb it is quite bright but as you get farther from the bulb it appears to be more dim. Because you know the bulb is 100 watts, you can tell exactly how far away it is by how bright it appears to you. We know exactly the brightness of type IA supernova so their apparent brightness tells us how far away they are. The speed at which the supernova is moving away from us is determined by the way in which the wavelength of light from the supernova is shifted. This is similar to something very familiar to most people: the Doppler shift of sound. When a train blows its whistle you can tell if the train is moving toward you or away from you by how the pitch of the whistle changes. If the train is moving toward you, a higher pitch is heard and if the train is moving away from you, a lower pitch is heard, and faster speeds will change the pitch even more. The pitch changes because the wavelength and frequency of the sound wave is affected by the motion of the train. In the same way, the wavelength of light changes if a star is moving toward us or away from us, and the amount of change can tell us the speed at which the star is moving. Using the method of a standard candle and the wavelength shift of light we measure that distant supernova are moving away from us in such a way that is best explained by postulating that the rate at which the universe is expanding is increasing.

Although we don't know what the dark energy is we know of certain processes that would cause the effect that we see. The simplest proposal for the source of dark energy is something called a cosmological constant. The cosmological constant is associated with the energy of space itself, the vacuum energy. We know that empty space is not really empty but filled with quantum fields and virtual particles. These fields give space itself a non-zero amount of energy. In fact, given what we know about physics and what we know about the virtual particles that can form in empty space we can actually calculate what we would expect the energy of empty space to be and then compare that to the observed dark energy. When we do this calculation, we fail miserably. The calculated value is about 120 orders of magnitude larger than the observed value; that is the calculation is 10120 times larger than what we actually observe. Scientists have many unanswered questions about why the dark energy is so much less than our best calculation. It is almost assuredly due to some unknown physics. As it is, though, the known value of the cosmological constant is another prime example of the fine-tuning we see throughout the universe. If the strength of the dark energy were only slightly more than its current value, the acceleration of the universe would have occurred so early in its history and been so fast that stars and galaxies could not have formed and we would not be here.

There are other observations that are best explained by dark energy. For instance, measurements of the cosmic microwave background radiation and the curvature of the universe show us the amount of dark energy compared with the matter energy in the universe. As shown in the opening figure in this article dark energy makes up about 70% of the total energy density of the universe. (The estimate has an uncertainty and ranges from about 68% to about 73% of the total energy of the universe.) The dark matter makes up about 25% which means that all of the matter we actually know about, all elements in the periodic table, all quarks and leptons, make up only about 5% of the energy of the universe. That fact is nearly impossible to comprehend when you consider how much we have learned about the structure of the universe. Everything we know makes up only about 5% of the energy of the universe. That means the other 95% is still waiting to be discovered.

It seems that the designer of the universe has boundless creativity. Over and over again as physicists learn more about the universe, we find that we are just scratching the surface of what there is to learn. The mystery of dark energy is likely to take a long time to explore and explain. It's finely-tuned value may take even longer to understand and unravel. I predict that these problems should keep us physicists employed for at least a little while longer. The path to discovery is always fun, and filled with unexpected twists and turns.


  1. Hi Dr. Strauss, Thank you for this insightful article on dark energy and dark matter. I gave a talk to the RTB Austin, TX chapter monthly meeting last week and had a slide on Genesis 1:4-5 on how God separated light from darkness. Imagine that, darkness is a substance! It only took modern science about 400 years to discover that darkness was a substance. But there it was in the very first chapter of the Bible written about 3000 years ago. Everything that we see and can measure only make up 5% of the universe. But God can see all 100%!

  2. What are the implications of dark energy in the context of the "rapid inflation" period of the earliest seconds of the universe. Was there more then than now, did some concert to dark matter quickly. Is the rate of expansion at the furthest regions of the universe we can observe likely to be the rate at the time of the "inflation". Thanks