These 36 images are galaxies hosting two types of 'milestone marker' to measure cosmic distances and the expansion of the Universe, type Ia supernovae and a special type of star known as a cepheid variable. Image Credit: SCIENCE: NASA, ESA, Adam G. Riess (STScI, JHU)

Thirty years of Hubble Space Telescope galaxy observations have now delivered one of the most accurate estimates of the expansion rate of the Universe – and also tells us that something is fundamentally missing from our current understanding of the Universe.

As reported in The Astrophysical Journal, researchers using the veteran space telescope have estimated that the expansion rate of the Universe is 73 kilometers per second per megaparsec plus or minus 1. That means that if you look at an object 1 million parsecs (3.26 million light-years) away, the expansion of the universe would make it look like it is moving away from you at 73 kilometers per second (over 163,000 miles per hour).

The new data is now known with just over 1 percent uncertainty. However, the problem is that a completely different estimate of the expansion rate of the Universe just 400,000 years after the Big Bang estimates that the expansion is 67.5 kilometers per second per megaparsec plus or minus 0.5. The tension between the two measurements has just grown and grown in the last few years.

We do not know why the two numbers don’t match, and there is only a million-to-one chance that the tension between the two is a fluke. Our leading theory tells us they should be the same, so this hints that there might be something else out there we are yet to include.

"The Hubble constant is a very special number. It can be used to thread a needle from the past to the present for an end-to-end test of our understanding of the universe. This took a phenomenal amount of detailed work," a member of the team Dr. Licia Verde, a cosmologist at ICREA and the ICC-University of Barcelona, said in a statement.

The scientific collaboration is called Supernova, H0, for the Equation of State of Dark Energy (SHOES) where H0 is the Hubble constant, the value of the expansion rate of the Universe. They observed 42 supernovae milepost markers. These particular types of events happen about once per year, so Hubble has pretty much studied all that have happened in the last three decades.

"We have a complete sample of all the supernovae accessible to the Hubble telescope seen in the last 40 years," SHOES leader and Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and the Johns Hopkins University in Baltimore explained.

"This is what the Hubble Space Telescope was built to do, using the best techniques we know to do it. This is likely Hubble's magnum opus, because it would take another 30 years of Hubble's life to even double this sample size."

Over the next decade, astronomers will expand the approaches to study the expansion of the Universe, and new and upcoming observatories will hopefully provide enough data to make us understand what’s going on with the Universe that we are yet to comprehend.

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