Unlocking the Mysteries of Dark Energy: What We Know and Don't Know

Friday - January 19 2024, 12:08 UTC - 10 months ago

The universe is made up of 5% ordinary matter, 25% dark matter, and 70% dark energy. Dark energy is an unknown force that is causing the universe to expand at an accelerating rate. Scientists are still trying to understand its properties, and a recent study has shed new light on the nature of dark energy. By measuring the brightness of exploding stars, known as Type Ia supernovas, astronomers can estimate the distance to these objects and gain a better understanding of the universe.

What is the universe made of? This question has driven astronomers for hundreds of years. For the past quarter of a century, scientists have believed "normal" stuff like atoms and molecules that make up you, me, Earth, and nearly everything we can see only accounts for 5 percent of the universe. Another 25 percent is "dark matter," an unknown substance we can’t see but which we can detect through how it affects normal matter via gravity. The remaining 70 percent of the cosmos is made of "dark energy." Discovered in 1998, this is an unknown form of energy believed to be making the universe expand at an ever-increasing rate. In a new study, soon to be published in the Astronomical Journal, my colleagues and I have measured the properties of dark energy in more detail than ever before. Our results show it may be a hypothetical vacuum energy first proposed by Einstein—or it may be something stranger and more complicated that changes over time.

What Is Dark Energy? .

When Einstein developed the general theory of relativity over a century ago, he realized his equations showed the universe should either be expanding or shrinking. This seemed wrong to him, so he added a "cosmological constant"—a kind of energy inherent in empty space—to balance out the force of gravity and keep the universe static. Later, when the work of Henrietta Swan Leavitt and Edwin Hubble showed the universe was indeed expanding, Einstein did away with the cosmological constant, calling it his "greatest mistake." However, in 1998, two teams of researchers found the expansion of the universe was actually accelerating. This implies that something quite similar to Einstein’s cosmological constant may exist after all—something we now call dark energy. Since those initial measurements, we’ve been using supernovas and other probes to measure the nature of dark energy. Until now, these results have shown the density of dark energy in the universe appears to be constant. This means the strength of dark energy remains the same, even as the universe grows—it doesn’t seem to be spread more thinly as the universe gets bigger. We measure this with a number called w. Einstein’s cosmological constant in effect set w to –1, and earlier observations have suggested this was about right.

Exploding Stars as Cosmic Measuring Sticks .

How do we measure what is in the universe and how fast it is growing? We don’t have enormous tape measures or giant scales, so instead we use "standard candles": objects in space whose brightness we know. Imagine it is night, and you are standing on a long road with a few light poles. These poles all have the same light bulb, but the poles further away are fainter than the nearby ones. This is because light fades proportionately to distance. If we know the power of the bulb and can measure how bright the bulb appears to be, we can calculate the distance to the light pole. For astronomers, a common cosmic light bulb is a kind of exploding star called a Type Ia supernova. These are white dwarf stars which often suck in matter from a neighboring star and grow until they reach 1.44 times the mass of our sun, at which point they explode. But all Type Ia supernovas reach pretty much the same maximum brightness, so by measuring the apparent brightness of one of these objects we can figure out how far away it is.