Overview
The standard cosmological model is facing a serious challenge. When observations of the infant Universe are combined with the physics of the Big Bang, the model predicts that the Universe today should be expanding at 67 km/s/Mpc. Yet direct observations of the nearby Universe consistently suggest a significantly faster rate of around 73 km/s/Mpc, implying a younger universe. This discrepancy, known as the Hubble crisis, has become one of the most important problems in modern cosmology.
Lynn’s Review
Many of you may have been wondering what I (who failed RSA Arithmetic over 60 years ago) will write as my review this month, for how can I possibly assess, or even give commentary to a presentation by Dr Banik. His knowledge, enthusiasm and passion for astrophysics took him into a dimension light years ahead of my humble ability to comprehend the equations used to form the hypothesis of the existence of a cosmic void.
We live in an amazing world; since man’s first appearance on planet Earth, he has questioned the how? Why? When? Of everything. And today, we still question by forming and testing out hypotheses. Here, I will set out what I understood from Dr Banik’s talk. There will be those of you who understood the equations presented, but here is my simplified version of the presentation, written after also, reading information presented by Dr Banik, and others, which is available on the internet.
First, I refer to a talk given to Science at Fishbourne in March 2024 by Dr Daniela Saadeh. At that time, it had been discovered that the rate of expansion of the Universe appeared to be accelerating and Dark Energy, which accounts for 68% of the Universe was possibly causing this increase in acceleration.
Dr Banik began his talk by explaining how the rate of expansion of the Universe is measured:
Standard Candles: Type 1a Supernovae are more massive than our Sun. They explode, when they run out of nuclear fuel, with a consistent intrinsic brightness. The brightness from pulsating stars, called Cepheids is also used.
Analysing the light from these astronomical bodies, as they travel through an expanding universe; looking at the spectoral wavelength of the photons, which will have been stretched by the time they reach us; the rate of expansion of the universe can be calculated.
As they move away from us, their light has lower frequency and shows as red. Closer objects will have light of higher frequency and show as blue. The red shift is used to calculate rate of expansion.
Cosmic Standard Rulers: comparing the true size of a cosmological object with its apparent size in the night sky will show its distance, and calculations can be made to show how space has stretched over time.
We know that the Early Universe was largely opaque and was 1100 times smaller than it is today. As it cooled stable atoms formed, and the opaque fog began to clear. Ancient photons of light were now free to travel across the Universe. This is what we know as: Cosmic Microwave Background Radiation – CMB. (From this point in time the Universe became observable). Its temperature is incredibly uniform. It has a frequency to it, and measurements of fluctuations can easily be made, so showing density differences in the early universe. Cosmologists can calculate the ingredients of the universe from this: normal matter, dark matter and dark energy. They can also calculate the distance and size of the universe and its expansion rate.
Baryon Acoustic Oscillations: In the first 380,000 years after the Big Bang, there was a continuous tug of war between gravity pulling matter inward and the radiation of photons pushing outward. Just like a pebble being dropped in a pond, the outward push created acoustic ripples; pressure waves, which continued until the universe cooled enough for neutral atoms to form and the universe became observable. These sound waves were now frozen in place.
The distance these waves were able to travel before freezing is known as the sound horizon, which spans approximately 490 million light-years (150 megaparsecs) in today's universe. As the`universe continued to expand, these frozen ripples dictated where matter accumulated; Galaxies formed in these denser regions and observations show a statistical preference for galaxies to be separated by about 500 million light years/ 150 megaparsecs. They act as a standard ruler, whose angular size can be used to chart the cosmic expansion history.
Knowing how the rate of expansion of the universe is measured cosmology is now in tension, some say: Crisis! The rate of expansion of the Universe is expressed by a quantity called: ‘The Hubble constant’ and the Hubble constant is about 73 kilometres per second/ per megaparsec (one megaparsec is just over three million lightyears). However, ESA’s Plank Mission measurements of the CMB, recorded a lower expansion rate of 67-68 kilometres per megaparsec. How can this be? And … Which measurement is nearer the truth? It looks as though the rate of expansion depends on where we measure it.
Dr Banik has been exploring the possibility that our galaxy is in the centre of an enormous void. The hubble constant applies in the distant universe. The Standard Model works and we can say that the age of the Universe rests at 13.8 billion years. In our near universe the rate of expansion is greater than predicted.
The cosmological principle, says that the universe’s matter should be evenly distributed on the largest scales, but the number count of galaxies in our local universe shows significant under density – about 20% lower than the average of the universe as a whole. Dr Banik in his research into BAO explained that by considering all available BAO measurements over the last 20 years, a void model is about one hundred million times more likely than a void-free model with parameters designed to fit the CMB observations taken by the Planck satellite.
So, the Earth and our solar system could be near the centre of a void about a billion light-years in radius. Quote Dr Banik: “The lower density of bodies inside the void means less gravitational pull, while the surrounding denser regions exert a stronger gravitational pull, so pulling objects within the void towards it.
A local void slightly distorts the relation between the BAO angular scale and the redshift, because the velocities induced by a local void and its gravitational effect slightly increase the redshift on top of that due to cosmic expansion alone.”
More research by looking into galaxies that are no longer forming stars, and by observing how the wavelength of photons is stretched, may help to shed more light on the history of the expansion of the universe.
I hope that my review makes sense. Below are links to content from Dr Indranil Banik, which you may like to look at.