The Universe Is Expanding Faster Than Your GPA Is Dropping (For Now)

While in the middle of AP season at Stuyvesant, it’s easy to forget that there’s a world beyond tests. In fact, while you were taking a three-hour AP Biology exam, our world, the universe, expanded 788,400 kilometers per megaparsec. Although this expansion has been accelerating over time, new findings suggest that the rate of acceleration might be changing, too.

The acceleration of the expansion of the universe is counterintuitive to the expectation that gravity would slow down the universe’s expansion over time. In the 1920s, Edwin Hubble observed that light from distant galaxies were redshifted, which suggested that the galaxies were moving away from Earth. A redshift is when a wavelength of light is stretched. As a result, the light is moved towards the red end of the visible light spectrum, where red light has the longest wavelength (700 nanometers). Through their observations of supernovae that were more redshifted than expected in 1998, astronomers observed that the universe was not just expanding but also accelerating.

The unknown force driving this acceleration is dark energy. Scientists do not know exactly what dark energy is and have not been able to conduct experiments to see dark energy’s effects because there are currently no tools to measure the universe at very large and precise scales. 

Although scientists cannot observe hundreds of millions of galaxies to measure or see dark energy, they know it makes up around 70 percent of the universe because scientists can track how fast galaxies with supernovae explosions from billions of years ago are moving. This helps scientists take into account how much dark energy is needed in order to estimate how the universe’s expansion has sped up over time. Scientists also know dark energy is uniformly distributed across the universe. They have been able to confirm their predictions for a uniformly distributed model of dark energy by comparing dark energy’s effects on hundreds of galaxy clusters through X-ray telescopes. 

The popular and famous cosmological constant theory states that dark energy is permeated by the vacuum of space. Little can be adjusted in Einstein’s general relativity without ruining its mathematical structure—except for a quantity Einstein coined the cosmological constant. The cosmological constant theory arises from that cosmological constant which Einstein invented to balance his theory of general relativity by canceling out gravity’s attraction, so his theory of general relativity features a static universe. Before inventing this constant, Einstein’s general relativity theory suggested that the universe was dynamic, which contradicted the common belief that the universe was static at that time. 

However, the cosmological constant has been reinterpreted in modern times to represent a mysterious form of energy or matter that acts in opposition to gravity, which is often considered equivalent to dark energy. Although this is a presumption because nobody knows exactly what the cosmological constant is, it is a quantity necessary in cosmological equations to match our observations of the universe. 

After scientists discovered that the universe’s expansion was accelerating and not static, some scientists suggested that the cosmological constant is a positive number that gives a just-noticeable effect—a number that is necessary to accelerate the universe’s expansion and attributes the force for the universe’s acceleration to the vacuum of space permeating dark energy, also called vacuum energy. So while you were hunched over your AP Biology exam on the verge of giving up, the universe did not stop. The universe raced apart, driven by a force more mysterious than the material you pounded your head trying to remember while taking the exam.

If this cosmological constant is indeed a constant, then the expansion of the universe would continue to accelerate unabated, leading galaxies to drift further apart at faster paces. However, new readings from the Dark Energy Spectroscopic Instrument (DESI)—mounted on the Mayall telescope in Arizona and designed to create the most detailed three-dimensional map of millions of galaxies in the universe—may change our understanding of dark energy.

DESI’s detailed map of the universe, combined with cosmic background radiation and supernova data, allows us to study how the patterns of baryon acoustic oscillations (BAO) patterns change over time. BAO patterns describe how events very early in the history of the universe have impacted the present distribution of matter. Studying the patterns of galaxies located closer to the Earth provides us with a sense of how the BAO patterns are distributed in the current universe, whereas investigating galactic distributions at large distances provides a mechanism to study the same patterns in the early universe. Together—by mapping out small-and large-scale patterns—researchers have developed a mechanism to understand how the universe has evolved. 

The observed data seems to suggest that dark energy is changing over time, therefore implying that the cosmological constant may not be constant. The effect of dark energy is, in fact, weakening over time, which means that the universe is expanding slower and slower. So, what does this change mean?

First, it might change the way the universe ends. Under the assumption that the cosmological constant is constant, researchers predicted that the universe would end in a heat death. The heat death model for the end of the universe predicts that, as matter is pulled farther and farther apart, the universe will cool down. Matter will decay and black holes will evaporate over time, ultimately leaving space empty.

However, DESI’s findings that dark energy might be weakening over time might mean that heat death isn’t the perfect model for how the universe will end. It opens up the possibility that cosmic acceleration could decrease to a standstill, eventually leaving the universe at a constant size and temperature. Another possibility is that the acceleration will decrease so much that it reverses, ultimately causing the universe to collapse on itself—a theory known as the “big crunch.” 

DESI’s findings are one of the first ways we’ve been able to map the universe and more deeply understand how dark energy changes over time. Ultimately, these findings will help refine existing theories of cosmology beyond the hypothetical question of how the universe will end, providing us with a deeper understanding of its workings.