Dark energy: what it is

15Oct10

Before I discuss the controversies surrounding dark energy and its introduction into the big bang paradigm, I figure I should make some kind of mention of what it is.

In my research I’ve come across, many times, the assertion that dark energy is the most famous and most embarrassing problem in physics. Maybe the most embarrassing, but the most famous? Really? I haven’t come across any non-astrophysicists who know what dark energy is; indeed, when I ever mention dark energy most people either have no idea what I’m talking about (the same goes for the fact that the universe expansion is accelerating, it’s big news to them!) or think I’m talking about dark matter (although, they mustn’t know much about dark matter either if that’s the case).

So… what exactly is it?

“We don’t know” seems like a good answer.

We call the force causing the universe’s expansion to accelerate “dark energy”. This information, that the expansion accelerates, has been confirmed by several independent cosmological probes, including measurements of supernovae , the cosmic microwave background, and galaxy clustering. So the acceleration itself generally is not in dispute, but the dark energy story gets muddy from here. The definition I use of dark energy is:

A property of empty space with negative pressure that counteracts gravity and causes the universe’s expansion to accelerate

Dark energy could be in the form of a cosmological constant, which is what many observations point towards, or it could be evolving in time as some type of quintessence model. Or it could be that general relativity is incorrect and dark energy doesn’t exist at all.

Besides the fact that there is a high level of uncertainty associated with dark energy, it seems to me that controversies arise in the dark energy story in three different ways:

  1. The initial supernovae measurements in 1998 that indicated that the universe’s expansion was accelerating
  2. Introducing dark energy as a cosmological constant (the Lambda-CDM model has come from this)
  3. It’s ugly

In the following posts I’ll cover each of these in more detail… I’m sure that those who are still reading will all be quivering in anticipation!

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