Expansion of the Universe
Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.
-Albert Einstein
Through years of human exploration, we have explored so many things about us and around us. We have crossed the boundary yet the answer to our question seems at bay. We might not always be right about our predictions or knowledge, but it has led us to some new insights and understanding of fields. The instinctive curiosity in us drives us to explore space and understand the laws that govern the universe.
Ancient people discovered the universe and celestial bodies for horoscopes and used zodiac signs to divide the heavens into twelve. This helped them to keep track of the position of the celestial bodies. As our knowledge of the universe expands we seek to learn more of the secrets it holds beyond our understanding. In addition, discoveries in the past have given us a more clear view of the universe with which we are able to predict the future.
Since then many astronomers have been predicting and proving their theories about the universe. From discovering that planets orbit around the sun to the big bang theory, we learned a lot about the universe. Similarly, astronomers have made discoveries about the expansion of the universe which has led us to predict the future of the universe.
It all started in 1912 when an astronomer named Vesto Slipher figured was asked to measure the doppler shift of the prominent spiral nebulae (now known to be galaxies). The results were expected to be random shifts of blue and red in equal numbers. But he realized that nearly all the nebulae exhibited a redshift. From his total list of 25 nebulae in 1917, 21 nebulae had a redshift.
Astronomers realized that this whole thing means that by the time galaxies were emitting light and it was reaching us, the galaxies were moving and stretching their wavelengths. This turned out to be one of the most important discoveries which helped astronomers in the future to predict the expansion of the universe. In 1927 another astronomer named George Lemaître predicted that these changing redshifts could only mean one thing: that the universe is expanding in all directions.
Then two years later an American astronomer Edwin Hubble studied this further and proved the prediction of Lemaître to be true. He determined the distance to many of the galaxies and stated that farther the galaxy is in deep space,Faster it is moving.
He found a more accurate but still not exact constant of the rate at which the universe is expanding which is known as the Hubble constant (Hₒ). He explained his constant and the expansion of the universe under Hubble’s law.
Now from expanding it doesn’t mean that it is moving away from us but in reality, there is more to it. The astronomers realized that it’s not the galaxies moving away through space, instead the space itself is expanding.
This phenomenon can be explained by taking an example of a polka dot balloon. When the balloon (universe) inflates, the dots (galaxies) move away from each other. Since all the parts of the universe are getting bigger, the farther the galaxies are from each other, the more the distance between them increases.
In 1931, Lemaître published a paper saying that initially, the physical universe was a single particle — “the primeval atom”- as he called it, which disintegrated into an explosion. This gave rise to space-time and the expansion of the universe. Many astronomers were uncomfortable with this idea as they believed that the universe is static and unchanging. This argument was ended when Arno Penzias and Robert Wilson discovered cosmic microwave background radiation in 1963. This is the first and still most important evidence of Lemaître’s theory which was later named as Big Bang.
Cosmic microwave background radiation helped to find out that the universe has been expanding for approximately 13.8 billion years. Through this discovery, astronomers got to know a lot about the past of the universe but the future of the universe is uncertain to predict. Cosmologists still don’t know if the universe will keep expanding forever or collapse one day.
Now according to the big bang theory, astronomers knew that the universe started very small but then it started expanding, and ever since it’s been expanding. But then they got the question: is the expansion of the universe getting faster or slower?
So in the mid-1990s, the group of cosmologists decided to find out the rate of expansion to know the fate of the universe. They realized through a survey held in 1989–1995 about supernovae (bright explosions from dying stars) that type 1a supernovae can be used to measure distances across space. They used the brightness or magnitude to measure the distance and its redshift helped to calculate the relative speed to earth. By this, they aimed to calculate the rate at which the universe is expanding and it was expected to be slowing down. Exactly how fast it was happening was the key to deciding if the universe is light or heavy.
However, when they looked beyond five billion light-years, they realized that the expansion is not slowing down. After re-confirming the results, again and again, the team went public in 1998 and changed the whole idea of expansion of the universe. Using Einstein’s equation of general relativity, they found that the universe has negative mass. In other words, there is an anti-gravitational force acting and the mysterious source of energy was named dark energy.
They knew that the universe is accelerating and it’s because of dark energy but they didn’t know what exactly dark energy is. So they tried to measure dark energy using one mathematical constant introduced by Einstein in his theory of gravity called the cosmological constant (Λ).
He used this constant in his general relativity equation where it acts against gravity pull and makes the universe an unchanging place. However, Einstein’s equation showed that the universe can only be dynamic, meaning it will either expand or contract. So he dropped this idea of the cosmological constant.
Cosmologists have tried to make some approximate measurement of how much dark energy there would be or would have changed over time. So far, all the evidence is consistent with a uniform energy density that extends throughout space. However, in terms of the equations of general relativity, the cosmological constant is equivalent to the non-zero value. So, in the standard version of Cosmology, called the Lambda-CDM model, cosmologists reintroduce the old concept of a cosmological constant in order to model dark energy. In other words, dark energy is either the same or somewhat similar to the presence of cosmological constant in the general relativity equation.
While finding the whole concept of expansion of the universe, there was one crisis in cosmology which was named Hubble tension. Based on supernovae as standard candles, the value of the Hubble constant (Hₒ) is around 73 km/s/Mpc. In the late 1990s, the lambda cold dark matter (ΛCDM) model was discovered which is looking at a small universe through a cosmic microwave background (CMB). Using the parameter within the model, cosmologists interpreted the rate at which the universe is expanding. The value of the Hubble constant using this method was around 67 km/sec/Mpc. The difference between both the measurements is more than 5σ. Hubble tension is still one of the most intriguing problems in cosmology today.
So far astronomers and cosmologists have learned a lot about the universe but the prediction of the future of the universe is still uncertain. There have been many theories based on the critical value of universe density. The cosmologist’s prediction of the future of the universe relies on the comparison of critical value and the average density of the universe.
There are three possible scenarios:
- A closed universe: when the average density is more than the critical value, the force of gravity will be enough to stop the expansion of the universe and reverse it. This can lead to a big crunch.
- An open universe: when the average density is less than the critical value, the universe will keep expanding forever and all the galaxies and stars will exhaust their energy. This will lead the universe to cool down and end in heat death/big freeze.
- A flat universe: when the average density is equal to the critical value, in this the gravity will be enough to stop the expansion but only after an infinite time.
We don’t really know what exactly the future of the universe holds for us. But thanks to the ancient astronomers we have learned a lot about the universe and are helping us to know further about the universe. Astronomers and cosmologists have been making their assumptions which have led us to many possibilities of our future and a better understanding of the universe.
We’ll go to our capacity to understand the universe but as Neil deGrasse Tyson said “The universe is under no obligation to make sense to you”.
Doppler effect: displacement of spectral lines occurs when you’re looking at an object which is moving towards or away from you.
Amitchell125
Redshift: By the galaxy getting redshifted means that the wavelength of visible light was being shifted towards the right side of the spectrum (red color). So for example, the nearby galaxy in the orange part of the spectrum will look redder by the time it gets to the telescope.
Hubble’s Law: Hubble’s law states that all objects moving away from each other have recession velocity which is proportional to distance. In other words, galaxies that are farther away from us are speeding away more quickly. V = Hₒd, the ‘H₀’ here is called Hubble’s constant but it’s not really constant because it will change depending on the evolution of the universe. That’s why there is sub-zero(0) which indicates that it is Hubble’s constant right now. The distance here is measured in Megaparsec (Mpc). The recession velocity can be determined by its redshift. The latest Hubble’s constant is 73.5 ± 1.4 km/sec/Mpc, Mpc is 3,260,000 light-years. We can use this equation to find the distance of any distant galaxy in Megaparsecs.
CMB: It’s the heat or the radiation left after the big bang happened.
Lambda CDM model: Lambda is associated with dark energy, CDM stands for cold dark matter. It is a cosmological model that describes the universe in terms of expansion, matter, and energy content and the relation between those.
Critical Value of universe density: It’s the density of matter in the universe required for the universe to be at a halt.
