However, the estimated size of this redshift drift over a decade is only about 10 cm/s. The redshift of the spectra of distant objects is an indication of the expansion of the Universe, so the change in this redshift over time is a measure of the change in the rate of expansion. This approach is known as redshift drift, and is a method that offers a truly independent and unique approach to exploring the expansion history of the Universe. A model-independent approach that measures the expansion rate directly was proposed as early as the 1960s, but limitations in technology have meant that astronomers have not been able to make such a measurement in practice. The ELT will contribute to current efforts to measure the acceleration of the Universe by characterising high-redshift Type Ia supernovas identified by James Webb Space Telescope and other survey facilities.īut the ESO telescope will also map the expansion history of the Universe using a whole different method, by watching the Universe expand in real time.Įxtracting information about the Universe’s expansion from standard probes relies on assumptions about the curvature of space, depends on the adopted cosmological model, and can only estimate the average expansion history over long time periods. Standard probes of the expansion of the Universe include weak gravitational lensing and the signature that light imprinted shortly after the Big Bang on today’s distribution of galaxies, and type Ia supernovas. The ELT will provide unique contributions towards deciding whether general relativity or a modified theory of galaxy best describes the expansion of the Universe, including testing the behaviour of gravity in unexplored regimes, as well as mapping the expansion history of the universe. Alternatively, it has been proposed that general relativity should be replaced with a modified theory of gravity, which would explain the accelerating expansion and the formation of structures in the early Universe by a different behaviour of gravity on the largest scales. As it happens, the theory of general relativity with the cosmological constant actually explains the accelerating expansion very well.
Ironically, the simplest form of such a dark energy is the cosmological constant that was introduced by Einstein when he was trying to ensure that his theory of general relativity allowed for a non-expanding Universe, the accepted view at the time. A key task for the next generation of astrophysical facilities is to search for, identify and ultimately characterise this new physics. It suggests our theories of cosmology and particle physics are incomplete (or possibly incorrect) and that new physics is out there, waiting to be discovered. We have named this component dark energy, but it remains a mystery. This discovery profoundly changed cosmology because it suggested that there is another component in the Universe that we cannot see, but that is acting against gravity and pushing space apart. However, in the 1990s a team of astronomers made the shocking discovery that the expansion is actually accelerating. For a long time, this expansion was assumed to be slowing down due to the gravitational pull that all matter in the Universe exerts on all other matter. In the 1920s, astronomer Edwin Hubble made revolutionary observations that provided the first direct evidence that the Universe was expanding, or that other galaxies are moving away from us and away from each other. The ELT will be able to peer at the dark matter halos around distant galaxies, to help us understand how much of each galaxy is made up of dark matter, and perhaps finally understand what exactly this strange substance is made of.ĭark matter makes up around a quarter of the Universe, but the even more mysterious dark energy makes up about 70%.
This led astronomers to believe that galaxies are surrounded by a halo of invisible dark matter, but despite being suggested almost 100 years ago, the nature of this dark matter remains enigmatic. So fast that the gravity generated by stars, dust and gas could not possibly hold them together.
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