How do scientists know that dark energy is speeding up the expansion of the universe?

One of the great conundrums in astronomy is the nature of something called dark energy, a kind of anti-gravitational force that appears to be pushing the accelerator pedal of the universe. It has been known since long that the universe was born about 13.7 billion years ago and has been expanding ever since. Yet until quite recently scientists thought that the rate of expansion was slowing down. In the mid-1990s, they found to their amazement that they were utterly wrong. The universe was speeding up and still continues to expand at an accelerated rate.

The discovery was made quite accidentally. Two teams of astronomers were measuring the brightness of very distinct Type 1a supernovae. These stellar explosions are among the brightest events in the universe, which makes them an ideal tool for determining their distances. The light from a Type 1a supernova follows a predictable path, always peaking at the same level of brightness. The light itself has a specific attribute. Its intensity varies inversely (oppositely) with the square of the distance between the source such as a supernova and the surface such as a telescope’s mirror.

In other words, if the distance increases, the illumination decreases by the square of the distance. To cite an example, if a surface that receives 1 lux of light at a distance of 1 meter from the source is moved 2 meters from the source, that surface will receive 1/4 lux of light. Astronomers, who were using supernovae as ‘standard candles’ discovered that every remote Type 1a supernovae were significantly fainter than they should have been, based on their distances estimated using other techniques. This meant that these supernovae were farther away than originally thought, so the implication was that the rate of expansion of the universe must be accelerating.

We don’t know that exactly is the repulsive force that is causing the universe to expand, so it has been called dark energy, and yet astronomers are totally in the dark about its physical nature.

Additional reading:
Supernova (Wikipedia)
Dark energy (Wikipedia)
Accelerating universe (Wikipedia)
Metric expansion of space (Wikipedia

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