The Crisis in Cosmology
Oh it’s a crisis alright…
What is Cosmology?
Cosmology is a branch of astronomy that deals with the origin, nature, evolution, and the ultimate fate of the Universe. It also involves studying exotic concepts like dark energy, dark matter, string theory, inflation theory, the concept of a multiverse, etc. Cosmology essentially covers the entire lifespan of the Universe. One of the most interesting facts about our universe is that it is expanding at an increasing rate. Cosmologists studying the expansion, and by extension the origins of the universe, were naturally led to ask the question: How old is our Universe? The answer to this question, however, isn’t quite as simple as one might expect.
The Problem
Hubble’s constant Ho, named after astronomer Edwin Hubble, expresses the rate at which the universe is expanding (Hubble, 1929). Knowing this rate of expansion helps cosmologists to then determine the age of the universe. The “crisis” in cosmology has arisen because the values for this constant, obtained from cosmologists best methods, are not in agreement (Di Valentino et al., 2019). In fact, the discrepancy between the values means there could be a difference in the age of the universe of about a billion years!
Method #1
The first and more indirect method used was studying the Cosmic Microwave Background Radiation or CMBR. The CMBR is the leftover electromagnetic radiation from the Big Bang that permeates all of space. Cosmologists picked an imprint of the early universe from a time soon after the Big Bang and compared that to a present model to predict the age of the universe that would lead to this evolution. (Planck Collaboration et al. 2018). This gives 67.4 ± 0.5 kilometers per second per megaparsec as the value for the Hubble constant and the universe an age of about 13.8 billion years.
Method #2
The second method, which has a more direct approach to calculating the value of the Hubble constant involves the use of variable stars like Cepheid. The variability of such stars can help determine the distance to nearby galaxies which in turn is used to measure the distances to galaxies further away, forming a type of distance ladder. For galaxies that are too far away, cosmologists use Type 1A supernovas instead. Researchers work out the distance by comparing the intrinsic brightness of the supernovas to the brightness observed by telescopes (Reid et al., 2019). With this method, the Hubble constant value was found to be 73.2 ± 1.3 km/s per megaparsec which gives about 14.5 billion years as the age of the universe.
Alternate Explanations
The difference between the two values is considerable. Over the years the gap between these two values has only increased as observational instruments got more refined, reducing the margin of error. Cosmologists have two possible explanations for this discrepancy.
The first possibility that has to be considered is that there is an error in measurements or data (Roettiger et al., 1997). The other and more existing possibility for cosmologists is that there is a gap in our understanding of the universe and the standard model. This could lead to a better understanding of dark energy and dark matter and the role it plays in the expansion of our universe or any number of other things that cosmologists haven’t even considered yet.
While some cosmologists have looked for these errors and created models to fill in the gaps, others have come up with new ways of measuring distances that are completely independent of the old theories and observations and can therefore provide more insight into the cause of the discrepancies.
One of the methods being looked into involves studying the gravitational waves from neutron star-neutron star mergers and neutron-star-black-hole mergers. These mergers not only result in the formation of gravitational waves but also light. Cosmologists can use this data to determine distances to galaxies completely independent of the distance ladder method giving more accurate results. Over the next few years, cosmologists hope to spot several such mergers to help obtain a more precise value for the Hubble constant. (Feeney et al., 2021)
While a simple solution may involve an error in instrumentation, the possibility of new physics being discovered is extremely exciting. Despite the concern over the discrepancies, one thing that we can say for sure is that this crisis will help cosmologists better understand the origins, composition, and evolution of the Universe.
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References:
[1] Hubble, E. (1929). A relation between distance and radial velocity among extra-galactic nebulae. Proceedings Of The National Academy Of Sciences, 15(3), 168–173. https://doi.org/10.1073/pnas.15.3.168
[2] Di Valentino, E., Melchiorri, A., & Silk, J. (2019). Planck evidence for a closed Universe and a possible crisis for cosmology. Nature Astronomy, 4(2), 196–203. https://doi.org/10.1038/s41550-019-0906-9
[3] Aghanim, N., Akrami, Y., Ashdown, M., Aumont, J., Baccigalupi, C., & Ballardini, M. et al. (2020). Planck 2018 results. Astronomy & Astrophysics, 641, A6. https://doi.org/10.1051/0004-6361/201833910
[4] Reid, M., Pesce, D., & Riess, A. (2019). An Improved Distance to NGC 4258 and Its Implications for the Hubble Constant. The Astrophysical Journal, 886(2), L27. https://doi.org/10.3847/2041-8213/ab552d
[5] Roettiger, K., Stone, J., & Mushotzky, R. (1997). Systematic Errors in the Hubble Constant Based upon Measurement of the Sunyaev‐Zeldovich Effect. The Astrophysical Journal, 482(2), 588–603. https://doi.org/10.1086/304176
[6] Feeney, S., Peiris, H., Nissanke, S., & Mortlock, D. (2021). Prospects for Measuring the Hubble Constant with Neutron-Star–Black-Hole Mergers. Physical Review Letters, 126(17). https://doi.org/10.1103/physrevlett.126.171102
