In the 1920s, Edwin Hubble and Georges Lemaitre made a startling discovery that forever changed our perception of the Universe. Upon observing galaxies beyond the Milky Way and measuring their spectra, they determined that the Universe was expanding. By the 1990s, with the help of the Hubble Space Telescope, scientists took the deepest images of the Universe to date and made another startling discovery: the rate of expansion is speeding up! This parameter, denoted by Lambda, is integral to the accepted model of cosmology, known as the Lambda Cold Dark Matter (LCDM) model.
Since then, attempts to measure distances have produced a discrepancy known as the “Hubble Tension.” While it was hoped that the James Webb Space Telescope (JWST) would resolve this “crisis in cosmology,” its observations have only deepened the mystery. This has led to several proposed resolutions, including the idea that there was an “Early Dark Energy” shortly after the Big Bang. In a recent paper, an international team of astrophysicists proposed a new solution based on an alternate theory of gravity that states that our galaxy is in the center of an “under-density.”
There are three known types of black holes in the Universe: supermassive black holes that lurk in the centers of galaxies, stellar-mass black holes that are the remnants of massive stars, and intermediate-mass black holes that can be found in dense clusters of stars. But there is a fourth, hypothetical type of black hole known as primordial black holes (PBHs). If they exist, they could solve a few cosmological mysteries.
The Universe has physical constants, such as the force of gravity that define everything. If these constants were any different, our Universe would look quite different. When you consider the types of objects that exist in our Universe – from quarks and bacteria to fleas and superclusters — different forces dominate their existence.
A fascinating new graph plots everything in the known Universe and shows us what’s possible. It also shows what types of objects are prohibited by the laws of physics as we understand them.
Fast Radio Bursts are an astrophysical enigma. They are intense bursts of radio energy lasting anywhere from a fraction of a millisecond to a few seconds, typically with a frequency of around 1,400 MHz, and we still don’t know what causes them. They were first detected in 2007 but were initially so rare and short-lived that it was difficult to confirm they weren’t terrestrial in origin. With the inauguration of the CHIME telescope and other wide-field radio observatories, we started observing lots of them, which confirmed they were both astrophysical and mostly coming from outside our galaxy. Now one has been observed from a galaxy 8 billion light years away, and it could help us solve a cosmological mystery.
Approximately 13.8 billion years ago, the greatest event in all of existence occurred that literally created existence itself. This event is known as the Big Bang, and it’s responsible for the estimated septillion number of stars that are scattered across the vast reaches of the unknown, including the one our small, blue world orbits. However, other than knowing that the Big Bang occurred, there is still a septillion amount of information we still don’t know about the greatest event in the history of existence.
Hydrogen is the most abundant element in the Universe. By far. More than 90% of the atoms in the Universe are hydrogen. Ten times the number of helium atoms, and a hundred times more than all other elements combined. It’s everywhere, from the water in our oceans to the earliest regions of the Cosmic Dawn. Fortunately for astronomers, all this neutral hydrogen can emit a faint emission line of radio light.
If you step outside during a moonless night and look up, it probably doesn’t look that dark at all. Streetlights or nearby porch lights fill the air with a background glow, particularly if they happen to be bluish-white LEDs. Light pollution in your neighborhood is likely so bad that you can only see a few bright stars. Even in somewhat rural areas, our skies are so bright that the Milky Way isn’t really visible. In North America and Europe, only about a quarter of children have seen the Milky Way.
The universe is filled with gravitational waves. We know this thanks to the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which recently announced the first observations of long wavelength gravitational waves rippling through the Milky Way. The waves are likely caused by the mergers of supermassive black holes, but can we prove it?
Since the 1990s, thanks to observations by the venerable Hubble Space Telescope (HST), astronomers have contemplated the mystery of cosmic expansion. While scientists have known about this since the late-1920s and early-30s, images acquired by Hubble‘s Ultra Deep Fields campaign revealed that the expansion has been accelerating for the past six billion years! This led scientists to reconsider Einstein’s theory that there is an unknown force in the Universe that “holds back gravity,” which he named the Cosmological Constant. To astronomers and cosmologists today, this force is known as “Dark Energy.”
However, not everyone is sold on the idea of Dark Energy, and some believe that cosmic expansion could mean there is a flaw in our understanding of gravity. In the near future, scientists will benefit from next-generation space telescopes to provide fresh insight into this mysterious force. These include the ESA’s Euclid mission, scheduled for launch this July, and NASA’s Nancy Grace Roman Space Telescope (RST), the direct successor to Hubble that will launch in May 2027. Once operational, these space telescopes will investigate these competing theories to see which holds up.
The first stars of the universe were very different than the stars we see today. They were made purely of hydrogen and helium, without heavier elements to help them generate energy in their core. As a result, they were likely hundreds of times more massive than the Sun. But some of the first stars may have been even stranger. In the early universe, dark matter could have been more concentrated than it is now, and it may have powered strange stellar objects known as dark stars.