In 1916, Einstein completed his theory of general relativity, which describes how gravitational forces alter the curvature of spacetime. Among other things, this theory predicted that the Universe is expanding, which was confirmed by the observations of Edwin Hubble in 1929. Since then, astronomers have looked further into space (and therefore, into the time) to measure how fast the Universe is expanding. – a.k.a. the Hubble constant. These measurements have become increasingly precise thanks to the discovery of the cosmic microwave background (CMB) and observatories such as the The Hubble Space Telescope.
Astronomers have traditionally done this in two ways: directly by measuring it locally (using variable stars and supernovae) and indirectly by relying on CMB redshift measurements and cosmological models. Unfortunately, these two methods have produced different values over the past decade. As a result, astronomers have been looking for a possible solution to this problem, known as the “Hubble Tension”. According to a new paper from a team of astrophysicists, the existence of “Early Dark Energy” could be the solution cosmologists have been looking for.
The study was conducted by Marc Kamionkowski, William R. Kenan, junior professor of physics and astronomy at Johns Hopkins University (JHU), and Adam G. Riess – astrophysicist and Bloomberg Professor Emeritus at JHU and the Space Telescope Science Institute (STScI). Their paper, titled “The Hubble Tension and Early Dark Energy”, is being reviewed for publication in the Annual Review of Nuclear and Particle Science (ARNP). As they explain in their article, there are two methods for measuring cosmic expansion.
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The direct method is to use supernovae as “standard candles” (distance markers) to make measurements at the local scale. The indirect method involves comparing CMB measurements with cosmological models – such as the Lambda Cold Dark Matter (LCMD) model, which includes the presence of dark matter and dark energy. Unfortunately, these two methods produce different results, the first giving a value of ~73 km/s per megaparsec (Mpc) and the second giving ~67 km/s Mpc. As Dr. Reiss explained to Universe Today via email:
“The Hubble constant is the current rate at which the Universe is expanding. The Hubble tension is a deviation in the value you find for the Hubble constant when you measure the rate of expansion as best you can at right now or you predict what value it should have based on how the Universe dealt with the Big Bang coupled with an evolutionary model of the Universe This is a problem because if these two ways don’t agree, it makes us think we misunderstand something about the Universe.
But as Reiss adds, the mystery of Hubble’s voltage isn’t so much a problem as an opportunity for new discovery. So far, many candidates have been proposed to explain the discrepancy, ranging from the existence of extra radiation, modified general relativity (GR), modified newtonian dynamics (MOND), primordial magnetic fields or the existence of dark matter and dark energy during the early universe which behaved in different ways. These can generally be divided into two categories: early solutions (soon after the Big Bang) and late solutions (more recent in cosmic history).
Late Solutions postulate that the energy density in the post-recombination Universe – when the ionized plasma of the early Universe gave rise to neutral atoms (about 300,000 years after the Big Bang) – is smaller than in the standard LCMB model. Early solutions, on the other hand, posit that the energy density has somehow been increased before recombination occurs, so that the “sound horizon” (the travel distance a sound wave can travel ) is decreased. For the purposes of their study, Kamionkowski and Kenan considered Early Dark Energy (EDE) as a potential candidate.
As Reiss explained, the presence of EDE would have contributed about 10% of the total energy density of the Universe before recombination occurred. After recombination, the energy density would have decreased faster than other forms of radiation, thus leaving the late evolution of the Universe unchanged. “This would produce an additional, unexpected burst of expansion in the young Universe which, if we were unaware of it, would lead to an underestimation of the predicted value compared to the true value,” Reiss said.
What makes EDE preferable to late solutions is the way the latter imply the existence of a fluid that actually creates energy out of nothing – which violates the strong energy condition predicted by GR. Moreover, these models are difficult to reconcile with cosmic distance scale measurements of Cepheid variables and Type Ia supernovae in nearby galaxies (low redshift targets) and Type Ia supernovae in distant galaxies (high redshift). In short, the solutions which involve modifications of the dynamics of the primitive Universe seem to be the most coherent with the established cosmological constraints.
As they note, although there is a growing body of evidence suggesting the existence of EDEs, our current CMB measurements are not yet sufficiently accurate and robust to distinguish EDE patterns from the standard LCDM pattern. What is needed, moving forward, are improved local measurements that will help refine the Hubble constant and remove any systematic error. Second, more accurate measurements of CMB polarization on smaller angular scales are needed to test EDE and other new physical models.
As they point out in their article, these measurements are already being taken thanks to the Dark Energy Survey observatories and next-generation observatories, such as the James Webb Space Telescope (JWST) and ESA Euclid assignment:
“Fortunately, the next steps in exploring Hubble’s voltage are clear. Moreover, the required observational infrastructure is already in place, as it largely coincides with that assembled to study dark energy and light. inflation (of the late Universe). Ultimately, we must continue to explore astrophysical and measurement uncertainties. As we have learned over and over again in cosmology, there is no one-size-fits-all solution – Solid conclusions are reached only with multiple lines of observation and a tightly knit web of calibrations, cross-calibrations, and consistency checks.
Further reading: arXiv
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