what-astronomers-wish-everyone-knew-about-dark-matter-and-dark-energy

What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy

Among the general public, people compare it to the aether, phlogiston, or epicycles. Yet almost all astronomers are certain: dark matter and dark energy exist. Here’s why.

If you go by what’s often reported in the news, you’d be under the impression that dark matter and dark energy are houses of cards just waiting to be blown down. Theorists are constantly exploring other options; individual galaxies and their satellites arguably favor some modification of gravity to dark matter; there are big controversies over just how fast the Universe is expanding, and the conclusions we’ve drawn from supernova data may need to be altered. Given that we’ve made mistaken assumptions in the past by presuming that the unseen Universe contained substances that simply weren’t there, from the aether to phlogiston, isn’t it a greater leap-of-faith to assume that 95% of the Universe is some invisible, unseen form of energy than it is to assume there’s just a flaw in the law of gravity?

The answer is a resounding, absolute no, according to almost all astronomers, astrophysicists, and cosmologists who study the Universe. Here’s why.

The expansion (or contraction) of space is a necessary consequence in a Universe that contains masses. But the rate of expansion and how it behaves over time is quantitatively dependent on what’s in your Universe. (NASA / WMAP science team)

Cosmology is the science of what the Universe is, how it came to be this way, what its fate is, and what it’s made up of. Originally, these questions were in the realms of poets, philosophers and theologians, but the 20th century brought these questions firmly into the realm of science. When Einstein put forth his theory of General Relativity, one of the first things that was realized is if you fill the space that makes up the Universe with any form of matter or energy, it immediately becomes unstable. If space contains matter and energy, it can expand or contract, but all static solutions are unstable. Once we measured the Hubble expansion of the Universe and discovered the leftover glow from the Big Bang in the form of the Cosmic Microwave Background, cosmology became a quest to measure two numbers: the expansion rate itself and how that rate changed over time. Measure those, and General Relativity tells you everything you could want to know about the Universe.

A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but is still expanding today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time. (Ned Wright, based on the latest data from Betoule et al. (2014))

These two numbers, known as H_0 and q_0, are called the Hubble parameter and the deceleration parameter, respectively. If you take a Universe that’s filled with stuff, and start it off expanding at a particular rate, you’d fully expect it to have those two major physical phenomena — gravitational attraction and the initial expansion — fight against each other. Depending on how it all turned out, the Universe ought to follow one of three paths:

  1. The Universe expands fast enough that even with all the matter and energy in the Universe, it can slow the expansion down but never reverse it. In this case, the Universe expands forever.
  2. The Universe begins expanding quickly, but there’s too much matter and energy. The expansion slows, comes to a halt, reverses, and the Universe eventually recollapses.
  3. Or, perhaps, the Universe — like the third bowl of porridge in Goldilocks — is just right. Perhaps the expansion rate and the amount of stuff in the Universe are perfectly balanced, with the expansion rate asymptoting to zero.

That last case can only occur if the energy density of the Universe equals some perfectly balanced value: the critical density.

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