A Cosmic Coincidence 13.8 Billion Years in the Making
Dark energy and dark matter densities are eerily similar right now - despite evolving at different rates over 13.8 billion years. Why do we happen to exist at the exact cosmic moment when they're equal? Coincidence seems unlikely.
老陳, CC BY-SA 4.0
The Universe's Recipe
The ordinary matter we see and interact with, stars, planets, and ourselves, makes up less than 5% of the universe. The rest is composed of two invisible components; about 27% is dark matter, a mysterious substance that provides the gravitational scaffolding for galaxies to form. The remaining 68% is dark energy, an even more mysterious force causing the expansion of the universe to accelerate.
Dark matter clumps together, and its gravity pulls things inward. Dark energy, conversely, is a repulsive force that pushes everything apart. For most of the universe's 13.8-billion-year history, these two forces have been wildly out of balance. Yet today, we find their energy densities are of the same order of magnitude. This observation, that we exist in the specific, brief epoch where these two dominant opposing forces are comparable, is known as the "cosmic coincidence problem."
A Tale of Two Densities
The coincidence is puzzling because the densities of dark matter and dark energy have evolved differently throughout cosmic history. In the early universe, space was much smaller. This made the density of dark matter, the amount of it packed into a given volume—enormously high. As the universe expanded, the dark matter spread out and its density dropped.
Dark energy, if it is a "cosmological constant" as theory suggests, behaves differently. Its energy density is an intrinsic property of space itself, so it remains constant over time. For the first several billion years after the Big Bang, dark matter's density was overwhelmingly dominant, and its gravity slowed the universe's expansion, allowing galaxies to form.
But as space continued to expand, dark matter's influence waned until, about five to six billion years ago, dark energy's constant repulsive force took over. At this tipping point, the universe's expansion stopped slowing down and began to accelerate. We live in the unique era where matter's gravitational pull and dark energy's cosmic push are nearly balanced. In the distant past, dark matter was everything; in the distant future, dark energy will be the only force that matters.
At an altitude of 2,200 meters in the Chilean Andes, the Vera C. Rubin Observatory will investigate this mystery. Its decade-long Legacy Survey of Space and Time (LSST) will map billions of galaxies, measuring the universe's expansion history and the growth of cosmic structures with unprecedented precision. This data will help scientists characterize the nature of dark energy and determine if this cosmic balance is a mere coincidence or a clue to physics.
💡Fun Facts
The term "dark energy" was coined in 1998 by cosmologist Michael Turner.
Albert Einstein first proposed a version of dark energy, the "cosmological constant," in 1917 to make his equations describe a static universe, later calling it his "biggest blunder."
If dark energy continues to accelerate expansion, in 100 billion years all galaxies outside our local group will be beyond the cosmic event horizon, making them forever invisible and undetectable from Earth.
The camera for the Vera C. Rubin Observatory is the largest ever constructed for astronomy, having 3,200 megapixels and being roughly the size of a small car.
Public visits to the nearby Cerro Tololo Inter-American Observatory are available on Saturdays, but require booking at least 24 hours in advance. The Vera C. Rubin Observatory also offers tours that fill up quickly.
Admission
Free, but reservations are mandatory.
Accessibility
The site is at a high altitude of 2,200 meters (7,200 feet). The tour requires travel on a mountain road. Visitors with hypertension or cardiovascular issues should take precautions.
