Hubble provides the most accurate measurement of the expansion of the universe to date

By analyzing more than 30 years of data collected by the Hubble Space Telescope, NASA was able to obtain the most accurate measurement yet of the universe’s rate of expansion. To operate, the researchers calibrated more than forty “kilometer markers” of space and time.

Imagine a chocolate chip muffin in an oven. The nuggets then move away from each other as the cake develops. If you could sit on one of these nuggets, you could watch everyone else drift away from you. In universe it’s a bit the same.

Astronomers have known for almost a century that the universe is expanding. The further away the galaxies are, the faster they travel. The speed at which they move relative to their distance from Earth is defined as Hubble constant. Measuring this value was also one of the main tasks of the telescope.

To measure the Hubble constant, astronomers analyze the distances of different types of objects whose luminosity is well known. Astronomers rely on it for relatively close objects the Cepheids. They are a class of stars that pulsate in a predictable pattern. Light off at greater distances Type Ia supernovaecosmic explosions of white dwarfs with a well defined peak brightness.

A more precise constant

Over the past few decades, measurements of these objects have allowed astronomers to calculate the Hubble constant at around 70 km per second per megaparsec (/s/MPC). Note that a galaxy one megaparsec (about 3.3 million light-years) from Earth is moving away at about 70 km per second, and that this speed increases by 70 km/s for each megaparsec.

However, in recent years teams have used other methods to try to refine this estimate, and the results have varied widely. For this new work, a NASA team compiled and analyzed the most complete catalog of these objects (Cepheids and Type Ia supernova) to date, in order to measure this constant as precisely as possible. All these objects isolated in forty-two galaxieswere mapped by Hubble above the last thirty years.

From this work, the team calculated the Hubble constant 73km/s/mpc, at plus or minus 1 km/s/Mpc. This reduces the uncertainty to just 1.4%, which is more accurate than other measurements. This new refinement could thus help astronomers improve models of cosmology and thus better specify its age or future fate.

Galaxy cluster Abell 2261 photographed by Hubble. Credits: NASA, ESA, M. Postman (STScI), T. Lauer (NOAO) and the CLASH team

A big gap that questions

However, a big mystery remains. The universe’s expansion rate has been predicted to be slower than what Hubble actually sees. By combining the Standard Cosmological Model of the Universe with measurements from the European Space Agency’s Planck mission (which observed the background radiation left behind by the Big Bang 13.8 billion years ago), astronomers predict a lower value for the Hubble constant: 67.5km/s/Mpc.

According to the researchers, given Hubble’s large sample size, there is only a one-in-a-million chance that astronomers will be wrong with the new estimate. However, the measurement of the Standard Model of Cosmology is also very strong. Astronomers are still at a loss to explain the discrepancy between the expansion rates of the local Universe and those of the early Universe, but the answer may involve something new physics.

The answer to this mystery may come our way with the James Webb Telescope, which will extend Hubble’s work by isolating cosmic marks at greater distances or with sharper resolution.

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