Dark Energy and the Runaway Universe

Image Description: DESI instrumentation team (i.r ‘DESI Collection, Dark Energy Spectroscopic Instrument’)

In 1912, the American astronomer Vesto Slipher made a surprising discovery when studying light from distant galaxies. He found that the wavelengths of this light were stretched, or redshifted, indicating these galaxies were moving away from us. This effect is similar to how the sound of an ambulance would become increasingly subdued as it moves away, a phenomenon called the Doppler Effect, except here it involves light rather than sound.

Seventeen years later, in 1929, Edwin Hubble built upon Slipher’s observations of redshift with his own measurements of how far away different galaxies were. To determine these distances, Hubble relied on special stars called Cepheids, which are known for regularly brightening and dimming in a predictable way, allowing him to accurately measure their distances. Hubble uncovered a straightforward relationship: the farther away a galaxy is from Earth, the faster it appears to be moving. This discovery, now known as Hubble’s Law, provided the first solid evidence that the universe itself is expanding – overturning the long-held belief in a static cosmos.

However, in 1917, Albert Einstein had already predicted something similar from his equations of General Relativity. Einstein’s equations naturally predicted an expanding or contracting universe, which went against the scientific consensus at the time. At that time, the majority of the scientists, Einstein included, believed the Universe to be unchanging. Unhappy with this outcome, Einstein introduced a mathematical “fix,” known as the cosmological constant, Λ (lambda). Its function was to essentially counteract gravity’s pull in his mathematical equations, forcing a static universe. Hubble’s discovery that the universe was indeed expanding led to Einstein later calling this fix his “biggest blunder.”

Fast forward to 1998, two independent research teams: the Supernova Cosmology Project, led by Saul Perlmutter, and the High-z Supernova Search Team, led by Brian Schmidt and Adam Riess, used bright stellar explosions known as Type Ia supernovae to measure how fast the universe was expanding at different points in history. Type Ia supernovae always reach approximately the same peak brightness, meaning astronomers could precisely calculate their distances. Both teams found these distant supernovae appeared dimmer than predicted if the expansion of the universe was slowing down due to gravity. These objects were dimmer because the universe had expanded more than expected while their light was on its way to us. In other words the expansion was accelerating! This acceleration is now attributed to something called dark energy. This discovery won the 2011 Nobel Prize in Physics.

Dark energy is nothing more than Einstein’s cosmological constant Λ redefined. It is a type of energy intrinsic to empty space or vacuum. In fact, Einstein’s original results had been correct and all that had to be done was to change the mathematical role of Λ from ‘cancelling’ the effects of gravity to instead amplifying them such that the Universe was now accelerating in its expansion rather than not expanding at all.

Unlike ordinary matter (such as stars or dust), we believe dark energy creates a repulsive effect, pulling space apart and accelerating the Universe’s expansion. Cosmologists measure the strength and behaviour of this energy using something called the equation-of-state parameter, symbolized as w, which describes how strongly something pushes or pulls on the universe. Current measurements from distant supernovae, background radiation left from the early universe, and patterns of galaxy distribution consistently suggest a constant dark energy.Or at least that is all we know as of now. Scientists still wonder whether dark energy has always been the same or if it could change over time. To explore this, new surveys like the Dark Energy Spectroscopic Instrument (DESI) are mapping millions of galaxies and quasars across the universe. DESI uses special features called baryon acoustic oscillations – fossilized ‘sound waves’ imprinted in the structure of the Universe – to track how cosmic expansion has evolved. Early results from DESI have already hinted that dark energy’s behaviour might be slightly different or changing, potentially pointing to new physics. Confirming this would profoundly reshape our understanding of how the universe evolves and the ultimate fate that awaits it.

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Article written by Mishita Khurana, an Integrated Astrophysics master’s student at the University of Edinburgh, interested in studying the large-scale structure of the Universe.

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Article edited by Eleanor Stamp, a Neuroscience PhD student at the Institute of Genetics and Cancer, University of Edinburgh, and an Online News Editor for EUSci. 

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And thank you to DESI for allowing the use of their image.