Jul 21 2016

Dark Matter and Dark Energy

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Genuine mysteries in science are fascinating, and there is no shortage of them. Scientists love mysteries because that is where the work is.

Two of the biggest scientific mysteries of our generation have similar names – dark matter and dark energy. Their names imply the unknown. They are, in fact, place-holder concepts that are temporarily representing what we don’t know. However, we are  slowly crawling toward an understanding of what they are.

Dark energy makes up about 70% of the mass/energy of the universe, while dark matter makes up another 25%, leaving just 5% for ordinary matter and energy. This means we currently don’t know what 95% of the universe is made of.

Fritz Zwicky first proposed the existence of dark matter in 1933, but his ideas were not accepted until the 1970s when they were revived by two astronomers, Vera S. Rubin and W. Kent Ford Jr. The hypothesis derives from the observation of how galaxies rotate. Their rate of rotation depends upon the amount of matter they contain – the more mass that exists within the orbit of any particular star, the faster that star will revolve about that galaxy. You can therefore estimate the amount of mass in a galaxy by observing how fast the stars are moving.

The problem is, when we look at galaxies the stars are moving faster than they should for the amount of matter we can see. There must be matter we cannot see – dark matter. This observation is now confirmed beyond reasonable doubt.

The only other possibility that has been raised is that the laws of gravity are not what we think. This notion is call modified Newtownian dynamics, or MOND. Perhaps gravity behaves differently are really large scales, like that of galaxies. This minority opinion has been essentially rejected by the evidence.

The two leading contenders for what dark matter might be are WIMPs and MACHOs. The former stands for weakly interacting massive particle and the latter for massive astrophysical compact halo object. These are not specific, just describing the type of matter that could explain dark matter.

WIMPs would be a new kind of particle, one that has mass, but otherwise does not interact much with other matter, and of course does not give off any detectable radiation. MACHOs could be ordinary matter, such as neutron stars or black holes, just matter that we can’t directly see.

The recent detections of gravitational waves from colliding black holes means that black holes may be much more common in the universe than we thought. It’s possible that black holes may have formed directly from collapsing clouds of gas in the early universe, rather than just as remnants of large stars. If that’s the case, all those extra black holes might explain a good chunk of dark matter.

In other words, dark matter may not be a new kind of matter, but just ordinary matter that we cannot see and that is not predicted by our current models of the universe. New evidence, however, may be altering those models and accounting for the missing matter. Time will tell. Dark matter is still in the unknown column, but we have some intriguing ideas.

Dark energy, in my opinion, is far more mysterious than dark matter. Again, this is a placeholder for the unknown, for something that needs to exist but we don’t know what it is.

In 1998 two teams of astronomers, observing type 1a supernovae, carefully calculated the rate of expansion of the universe over time. The prevailing hypothesis was that the universe would be slowing down. Whatever initial velocity it had from the big bang would be gradually slowing due to the mutual attraction of gravity. What cosmologists did not know was if the universe would just slow down forever asymptotically, or if it would slow down, stop, then start collapsing again, ending in a big crunch.

The answer, it turns out, is neither. The astronomers discovered, to their surprise, that the expansion of the universe is accelerating. This must mean that there is some energy pushing the universe apart, and this energy is stronger than the pull of gravity. We have no idea what this energy might be, hence the name, “dark” energy.

Interestingly, just as with dark matter, some scientists hypothesized that the expansion may be due to a modification in our understanding of gravity, specifically general relativity, rather than the existence of a new force.

As explained in a recent Science article:

In general relativity, given the distribution of mass and energy, spacetime bends to minimize its curvature, denoted R. But in so-called f(R) (pronounced “eff-of-are”) theories, spacetime contorts to minimize the curvature plus some extra function of the curvature. That change produces an extra gravity-like force that can either attract or repel under different conditions.

Essentially they argue that we just need to plug in the correct value for f(R) in the equations of general relativity, and that will result in the repulsive force that explains the acceleration of the universe, without the need to introduce dark energy.

Like all good scientific hypotheses, the f(R) hypothesis makes different predictions about what the universe should look like than the dark energy hypothesis. Modified f(R) should produce more massive clusters of galaxies than dark energy in the early universe.

Astronomers have recently made observations using a technique known as weak lensing. They are looking at how galaxy clusters bend the light from more distant objects. Using this technique to estimate the mass and number of galaxy clusters, astronomers so far have found numbers more consistent with dark energy than modified f(R).

So, for now, dark energy remains the leading contender to explain the expansion of the universe. This still leaves the mystery of what dark energy actually is.

Conclusion

In the last few centuries science has been remarkably successful in building models that explain and predict the behavior of the universe. We see the results of that success so often we take it for granted, such as the recent successful mission to Pluto. The fact that we were rewarded with close up high-res pictures of distant Pluto is a testament to the success of chemistry, physics, astronomy, and engineering. No other human endeavor enjoys such stunning confirmation.

At times this success tempts individuals to conclude that we are nearing the end of science where all the big questions have been answered. This was declared before Einstein changes our view of reality completely.

In 1996, Scientific American write, John Horgan, published the book, The End of Science. This was two years before the discovery of the acceleration of the universe and the hypothesis of dark energy.

Horgan still defends his thesis, writing in 2015:

So do I take anything back?

Hell no.

Aristotle’s theories were wrong and our theories are right. The Earth orbits the Sun, not vice versa, and our world is made not of earth, water, fire and air but of hydrogen, carbon and other elements that are in turn made of quarks and electrons.

In short, his notion is that science is finite. At some point we must pass “peak science” (that’s my term, not his, as far as I know), and scientists will be left to dot some “i”s and cross some “t”s but there will be nothing big left to discover.

That is an interesting question – will we ever get to that point, or is the universe sufficiently big and complex that effectively there is no bottom to our ignorance? The answer will likely be different for different fields. Biology may run out of big mysteries, until we find life on another planet. Once we can travel to different systems, we may never run out of new biology to explore.

Physics is different. I can see discovering all the forces and particles that exist at some point.

The other view is that Horgan is right and wrong. Some of our current answers are so solid they will never change: the Earth is not flat, DNA carries information of inheritance, the continents are moving, light is composed of photons. These answers will be the same in a thousand years and a million years of science.

As a science matures it no longer introduces revolutionary change, rendering prior knowledge wrong. Rather, it progresses by deepening our knowledge. Horgan dismisses this as details, but I think that view is wrong. Deeper may be finer in some respects, but it can also be more fundamental in others. Once we discover what all the particles that exist are, we may begin exploring why there are those particles and not other particles. Would that knowledge not be more profound?

We know all the fundamental forces (probably), but we have not unified all of them. In unifying the forces we may discover something much more fundamental about how the universe works.

When you consider the math, general relativity is a subtle tweak on Newtownian mechanics. Conceptually, however, it revolutionized our understanding of reality.

For now I would say the demise of science has been greatly exaggerated. At the very least it is premature. While it is an interesting question, I think it is also very premature to conclude whether or not science will ever end. At least we don’t have to worry about it as long as there are mysteries like dark energy and dark matter.

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