This tiny region of the JADES survey shows a mix of galaxies: some that are relatively nearby, large, highly evolved, and massive; others that are at intermediate distances and have a mix of old-and-young stars in them, and a great number of very distant or even ultra-distant galaxies that are faint, heavily reddened, and potentially from the first 5% of our cosmic history. In this one little region, the power of JWST, and the evolution of the angular scale of the Universe, is on full display. Note that all galaxies are equally sharp; the more distant galaxies exhibit no observable “blurring” compared to the nearer ones. (Credit: NASA, ESA, CSA, STScI)

For many years, cosmologists have claimed the Universe is 13.8 billion years old. A new paper says no, it’s 26.7 billion. How do we decide?

Ethan Siegel
Starts With A Bang!

Perhaps the greatest achievement of late-20th and early-21st century astrophysics is the arrival of a “consensus model” of the entire Universe: Lambda-CDM (ΛCDM) cosmology. For countless generations of humans, we’ve marveled at questions such as:

  • What is the Universe?
  • What makes it up?
  • How far, to the limits of what it’s possible to observe, does it go on for?
  • How did it come into existence, and how long ago?
  • How did it grow up to be the way it is today?
  • And what will its ultimate fate be?

Today, after unprecedented measurements of galaxies all throughout cosmic history, all-sky imaging of the Universe at microwave wavelengths, and thousands upon thousands of supernovae and other transient events all across the Universe, we finally have our answers to these questions. Our Universe, made of 68% dark energy, 27% dark matter, and just 5% “normal stuff,” began from a small, dense, nearly-perfectly-uniform state some 13.8 billion years ago in a hot Big Bang, and has been expanding, cooling, and gravitating ever since.

At least, that’s the consensus picture. Recently, a challenge to that picture has gotten some popular attention, based on a recently published paper claiming that the Universe is actually 26.7 billion years old, not 13.8 billion years old. Let’s look at these two theories side-by-side, and unpack what’s true versus what we’d need to be true in order to truly determine the age of the Universe.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago. (Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research)

Different assumptions

Anytime you’re given a scientific theory, you have to ask yourself, “what assumptions are behind it?” In the case of the standard model of cosmology, ΛCDM, the assumptions are that:

  • the laws of physics are given by General Relativity (for gravity) and…