Dark Energy

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Cosmic Acceleration

Our universe is a wonderfully surprising place.

Cosmic acceleration is a great example of why this is true. Until recently, it had always been assumed that the expansion of the universe was slowing down due to the effect of gravity. This would seem to be a natural and obvious conclusion to make. Amazingly, current observations indicate that the universe is actually expanding at an ever increasing pace. This unexpected observation came as a real shock to many cosmologists back in 1998 and really put a spanner in the works!

So what is causing this acceleration? What is pushing the universe apart? This is one of the greatest scientific mysteries we are currently faced with. At present, it is by far the most important unsolved problem in cosmology, as well as having important consequences for particle physics, as we shall see. By studying this modern acceleration epoch we can probe uncharted areas of physics, learning about quantum and gravitational physics, and perhaps even approach a unification of the two.

Dark Energy

Dark energy is a hypothetical form of energy that has been proposed to explain the cosmic expansion. Dark energy permeates all of space and has a strong negative pressure. In other words, it is a repulsive gravitational effect or anti-gravity, pushing the universe apart.

Roughly 70% of the universe mass/energy budget consists of dark energy. Credit NASA.

Recent evidence points to the universe being flat. But when astronomers measure the amount of matter and energy in the universe today, they only come up with about 30% of what is needed to make the universe flat. So assuming general relativity is valid at all length scales, about 70% of the energy density of the universe is in the form of dark energy. At present, the nature of dark energy is unknown. It is quite remarkable that such an important component of the universe remains so enigmatic.

It is unclear what relation, if any, exists between dark energy and inflation. Unlike inflation, dark energy does not currently completely dominate the universe.
There is no known connection between dark energy and dark matter, despite the similar names.

When the universe was younger and denser, the gravitational attraction was stronger. As the universe has expanded its density has decreased resulting in a decrease in the effect of gravity. About 4 to 5 billion years ago, the dark energy surpassed the effect of gravity, resulting in cosmic acceleration.

A number of mission proposals have been made to investigate dark energy -

Joint Efficient Dark-energy Investigation (JEDI)

The Dark Energy Survey

Theories of dark energy have proposed both a cosmological constant and a variable cosmological parameter.

What is the Evidence?

So how do we know the universe is accelerating? And how do we go about modeling such a strange phenomenon?

The evidence for this cosmic acceleration comes primarily from type 1a supernovae data. The acceleration has been confirmed using other cosmological probes such as the cosmic microwave background, gravitational lensing, the age of the universe and large scale structure.

Type Ia Supernova

Surveys use Type Ia supernovae as standard candles to determine the luminosity distance vs. redshift relation. The supernova survey technique is sensitive to dark energy through its effect on this relation.

These supernovae have the advantage of being extremely bright and so can be seen at cosmic distances. Type 1a supernovae are not equally luminous, but they may be calibrated using nearby supernovae according to their brightening and fading. This is achieved using Cepheid calibration of the peak brightness of type 1a supernovae.