Starts With A Bang

Is there really a cosmological constant? Or is dark energy changing with time? (Synopsis)

The history of the Universe tells the story of a race between gravitation and expansion, until about six billion years ago, when dark energy becomes important. Image credit: NASA / GSFC.

“If you’re puzzled by what dark energy is, you’re in good company.” -Saul Perlmutter

We normally assume that the fundamental constants of the Universe are actually constant, but they don’t have to be that way. They could vary in space, in time, or with the energy density of the Universe, in principle. Before believing in such an extraordinary claim, however, you’d need some remarkable evidence. It’s arguable that exactly that sort of evidence is emerging: from the tensions in the expansion rate of the Universe.

Three different types of measurements, distant stars and galaxies, the large scale structure of the Universe, and the fluctuations in the CMB, tell us the expansion history of the Universe. Image credits: NASA, ESA and the Hubble Heritage Team (STScI), SDSS, ESA and the Planck Collaboration.

If you measure the expansion rate from the cosmic microwave background, you get a value for the expansion rate of 67 km/s/Mpc. But if you measure it from the traditional cosmic distance ladder, you get a value closer to 74. This tension could be a systematic error in the measurement, but it could also point towards the value of dark energy changing with time. Interestingly, a large survey independent of the Universe’s expansion but dependent on weak lensing shows an increasing dark energy might be the answer.

Any configuration of background points of light — stars, galaxies or clusters — will be distorted due to the effects of foreground mass via weak gravitational lensing. Even with random shape noise, the signature is unmistakeable. Image credit: Wikimedia Commons user TallJimbo.

It could all be systematic errors, of course, but if the effect is real, it could revolutionize how we understand the Universe. Sabine Hossenfelder explains.