Dark Energy

What Is Dark Energy?

In the late 1990s, astronomers found evidence that the expansion of the universe was not slowing down due to gravity as expected. Instead, the expansion speed was increasing. Something had to be powering this accelerating universe and, in part due to its unknown nature, this “something” was called dark energy.

Hubble plays an important role in verifying, characterizing and constraining dark energy. Both Hubble and ground-based observations measures a special type of stellar explosion, a white dwarf supernova, to measure accurate distances to galaxies. 

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A galaxy located a billion light-years away provides a data point for the universe as it was a billion years ago. Meanwhile, as the universe expands, the light traveling to Earth from distant galaxies (and their supernovas) is stretched out to longer wavelengths — a phenomenon called cosmological redshift. The cosmological redshifts of galaxies at different distances provides a history of the expansion of the universe over time. 

However, only Hubble had the resolution to extend these observations to very distant galaxies. The discovery of supernova 1997ff, located about 10 billion light-years away, provided evidence for dark energy. 

About halfway into the universe’s history — several billion years ago — dark energy became dominant and the expansion accelerated. While ground-based studies had measured this accelerating period, Hubble’s observation of 1997ff stretched back to the decelerating part of the expansion. This shift between two different eras of the universe — a change from a decelerating universe to an accelerating universe — showed that dark energy exists.

Hubble continued to explore the nature of dark energy with observations such as the Great Observatories Origins Deep Survey (GOODS), structured to help uncover distant supernovas.

The 42 supernovas found by Hubble not only solidified the conclusions about dark energy, but also began to constrain some of its possible explanations. Later Hubble results identified how early in the universe dark energy began to influence the expansion as well as constrained the current expansion rate.

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The view that emerged was that dark energy was consistent with the slow, steady force of Einstein’s cosmological constant, a concept that the physicist had initially introduced into his equations to prevent his theoretical universe from collapsing, then later retracted when the expansion of the universe was discovered. But instead of holding the universe in a steady state, dark energy is pushing outward to expand the universe faster and faster. The discovery of dark energy was recognized by the Nobel Prize in Physics in 2011.

Astronomers now know that there is much more to the universe than meets the eye. The luminous and non-luminous normal matter makes up about 4 percent of the total mass and energy density of the universe. Dark matter, which emits no light and cannot be directly observed, comprises another 24 percent of the total, while dark energy dominates with about 72 percent. Most of the universe is unknown and only indirectly detected. We can see its effects on galaxies and the expansion of the universe, but we have yet to identify the underlying source. That may seem unsettling, but to a scientist, it is exciting. There are more great mysteries to explore and solve!