For decades, astrophysicists have studied various aspects of the universe. From black holes to the origin of the universe itself. But some have gone far enough to contemplate the unknown or unknowable. Is it possible that something exists even beyond the universe?
To answer that question, we first have to define the universe. Most of the time the word ‘universe’ implies to the observable universe, a region with all the matter that can be observed from the Earth at present. It is defined by the speed of light.
The word ‘observable’ here doesn’t imply to the ability of modern instruments to detect light from a distant object, but the physical boundary bought up by the speed of light itself. Since nothing can travel faster than light, we cannot observe anything farther from the earth than the light could travel in the time the universe has existed (13.7 billion years).
According to the calculations, the diameter of the observable universe is believed to be about 93 billion light years or 28.5 gigaparsecs. Now you must be thinking how the universe can be 93 billion light years across if it’s just (relatively speaking) 13.8 years old?
Image Courtesy: Andrew Z. Colvin
Let’s break it down. According to the Hubble’s law, extremely distant regions expand faster than the speed of light, on the other hand, special relativity tell us that two close objects cannot move faster than the speed of light with respect to one another. So it is not the object that is traveling faster than the speed of light but the space between them.
So, if you travel faster than the speed of light in a straight line towards the edge of the universe, you will see the edge of the universe, right. Well, the answer is no because there is no edge. We know that our universe is isotropic and expanding, but expanding into what?
For one moment just imagine that our cosmos is actually one universe in a multiverse, where each universe is like a soap bubble fixed in the void of the multiverse. Each bubble is expanding from its own Big Bang and they have their own laws of physics too. This might sound crazy, but hang on, here we might have something interesting.
If by any chance, two of these bubble universes get close enough and start interacting with each other in such a way that second universe is detectable from within the first universe, we can have proof of this hypothesis. Astronomers have actually looked out into space in search of any sign that might tell us that our universe is interacting with another universe and guess what, they have found something really peculiar.
After surveying the cosmic microwave background or CMB, scientists have spotted temperature fluctuations in various regions. While most of these temperature fluctuations are explained by contemporary cosmological theories, there is one region, known as the “Cold Spot” that disobey all the existing theories.
For some weird reason, the “cold spot” is about 70 µK (microkelvin) colder than the average temperature of 2.7 k. While it might not be a problem in a small region, but this covers a huge region, which is one billion light years across. It is located in the southern celestial hemisphere (don’t confuse it with Earth’s southern hemisphere) in the direction of Eridanus constellation. Many researchers have named it “The Cosmic Axis of Evil“.
The CMB Cold Spot in circle
To explain the “Cold Spot”, astronomers have come up with few theories or hypothesis. One such hypothesis is the Supervoid hypothesis, which suggests that this anomaly is caused by the presence of a huge void between us and CMB in that direction. Another possible explanation is the Cosmic Texture hypothesis.
But the most fantastic and most controversial of all is that the “Cold Spot” is actually caused due to the collision between our universe and the other universe and the spot is precisely where its interacting. This hypothesis have also gained some ground recently.
In 2017, Professor Tom Shanks of the Durham University in his paper stated that a supervoid is highly unlikely to explain a “Cold Spot” of that size. He added, while we cannot rule out other explanations provided by the standard model, in a situation where they too fail to provide the answers, we have to consider more exotic explanation – a collision between our universe and another bubble universe.