Hubble trouble: Constant tension – with Professor Daniel Mortlock

Veil Nebula from Hubble

public in-person lecture

A century ago, Edwin Hubble discovered that the Universe is expanding and made the first measurement of its rate, now known as the Hubble constant. Professor Mortlock divulges the current ‘Hubble tension’ about its conflicting measurements stirring the astrophysics world.

Daniel Mortlock - Professor of Astrophysics and Statistics, Physics and Mathematics departments, Imperial College London. 



Edwin Hubble was an American Astronomer who changed our view of the Universe. In 1929 he showed that galaxies are moving away from us with a speed proportional to their distance. The explanation was simple, but revolutionary: the Universe is expanding. He changed the way we thought of the Universe forever.

He made the first measurement of what is now known as the Hubble constant, H0, which characterises the current expansion rate.

Despite being conceptually a very simple quantity, the near impossibility of making precise and reliable distance measurements of extra-Galactic objects (i.e. outside our own Milky Way galaxy) has made it extremely difficult to determine the value of H0.

After a century of sometimes vitriolic debate about this, in the early 2000’s the Hubble Space Telescope Key Project seemed to have settled the issue, with H0 established to 10% precision at around 70 km/s/Mpc (kilometres per second per megaparsec, somewhat baroque units which will be defined in the talk).

However, in the time since, a “tension” has arisen between local “direct” measurements of H0, which imply H0 = 73+/-1 km/s/Mpc, and “cosmological” measurements from the cosmic microwave background (CMB) radiation, which imply H0 = 67+/-0 km/s/Mpc. The reported uncertainties on both measurements are too small for this discrepancy to be random chance, implying either i) new physics or ii) a significant re-evaluation of the data will be required to resolve this “Hubble trouble”.

Professor Mortlock grew up in Melbourne, Australia, where he did first an undergraduate degree and then a PhD in physics at the University of Melbourne. He then moved to the UK, working at the University of Cambridge and then Imperial, where he has been a permanent member of staff since 2011. His astrophysics research includes the search for the most distant quasars, measuring the Hubble constant and galaxy evolution.

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  • South Kensington Campus,
  • Imperial College London
  • London SW7 2BX

Event Schedule

  • 30 January 2025 7:00 pm   -   8:30 pm
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