Diving Into The Void
Is the universe stationary? Is it expanding? If yes, then at what speed? What is the element that is responsible for this? Let's crack it down bit by bit!
We've come a long way from 1922 when Friedmann proposed his model of the universe. While proposing the idea for his models, Friedmann made two assumptions: that the universe looks identical in whichever direction we look and that this would be true if we were observing the universe from anywhere else. The first assumption has been proved by two American physicists in 1965, Arno Penzias and Robert Wilson; we'll look at their work probably in the coming article. However, there is no proof to support or deny the second assumption; we accept it simply on the grounds of modesty. Although Friedmann could propose only one model based on his assumption, there are, in fact, three different kinds of models that obey his two fundamental assumptions. In the first kind, which Friedmann found, the universe is expanding so sufficiently slowly that the gravitational attraction between different galaxies causes the expansion to slow down and eventually to stop. The galaxies then start to move towards each other and the universe contracts. The distance between two galaxies starts at zero, increases to a maximum, and starts decreasing down to zero again.
In the second kind, the universe expands so rapidly that gravitational attraction is not enough to stop it, though it slows the process a bit. The separation between galaxies starts at zero and keeps on increasing at a steady rate.
Finally, in the third model, the universe is moving just fast enough to avoid the recollapse. In this case, too, the separation starts at zero and keeps on increasing forever, although the speed of expansion gets smaller and smaller but never reaches zero.
To determine whether the universe is expanding or contracting we will have to know the present rate of expansion and its present average density. If the average density is less than a certain critical value determined by the rate of expansion then the gravity will be too weak to halt the expansion, on the other hand, if the density is greater than the critical value then the gravity will stop the expansion at some point in the future and will force it to recollapse.
We can determine the present rate of expansion by measuring the velocities at which other galaxies are moving away from us, using the Doppler effect. This can be done very accurately. However, the distance to other galaxies is not very well known because we can only measure them indirectly. From this, we conclude that the universe is expanding by between 5-10% every thousand million years. However, our uncertainty about the present average density of the universe is even greater.
If we add up the masses of all the stars in our galaxy and other galaxies, the total is less than one-hundredth of the amount required to halt the expansion of the universe. At this point, we introduce the concept of dark matter which we cannot see directly, but which we know must be there because of the influence of its gravitational attraction on the orbits of stars and gas in the galaxies. Even after adding up the mass of all the dark matter, we would still get just one-tenth of the amount required to halt the expansion. There might be some other matter that we have not detected yet, however, the present evidence suggests that the universe will probably expand forever.
To explain the theoretical model of the universe scientists has come up with the composition of the universe which is ~68% dark energy, ~27% dark matter, ~5% normal matter. So what is dark matter?
Let’s first understand what is meant by dark here? Dark things absorb light/wavelengths of all sources but never emit anything detectable of their own. It blocks all forms of wavelengths. To clear the confusion a black body is different than a dark body in the sense that it absorbs all wavelengths and also emits electromagnetic radiation based on its temperature.
To break it down further; dark matter, first of all, is dark which means it is not in the form of stars and planets that we see. Second, it is not in the form of dark clouds of normal matter (matter made up of baryons) we know this because we would have detected baryonic clouds by their absorption of radiation passing through them. Third, it’s not antimatter because we don't see gamma radiation when antimatter-matter annihilation takes place. Finally, we can rule out black holes which will make a fascinating article in itself (Are black holes dark matter?) for now we can say that based on the number of gravitational lenses we see and the property of "normal” matter to bend light passing near it which comes from a distant object confirms that dark matter is not black holes. It is made up of some other, more exotic particles like axions or WIMPS-Weakly Interacting Massive Particles. To be exact this dark matter is purely “invisible” to everything in the universe, matter, energy, other particles can simply go past it without acknowledging its presence out there in the void. Then how do we detect dark matter? Well, to say the least on this subject, it’s the influence that it has on the orbits of stars and other objects in the galaxy which propose a need of introducing dark matter in the story, we will of course look into this in detail in the upcoming articles.
So now we are familiar with the expanding universe which hasn't slowed as scientists thought it would but accelerating, no one knew how to explain this expansion and the cause behind it. Eventually, theorists came with different explanations. These explanations included a long-discarded version of Einstein's theory of gravity, the one that contained what was called a "cosmological constant". Or maybe there was a kind of energy-fluid that filled the space or maybe there was something wrong with Einstein's theory of gravity and a new theory could explain this "cosmic acceleration" due to some kind of field. Theorists didn't come up with the ultimate solution but they sure gave this solution the name "dark energy”.
More is unknown about this dark energy than is known. What we do know is how much dark energy there is because we know how it affects the universe’s expansion. We have already covered the composition of the universe above. Dark energy is responsible for almost 68% of the total, dark matter makes up 27% and everything that we have observed with our eyes or telescopes or any other instruments makes up for less than 5% which is called normal matter. Come to think of it, maybe it shouldn’t be called normal at all.
Now comes my favorite part- “Empty Space is not Nothing". Einstein was the first to realize this. Space has stupendous properties, likewise, dark energy is one of those properties. The first property that Einstein realized was that more space can come into existence. Then Einstein's theory of gravity, the one which contains a cosmological constant makes a second prediction: "space" can have its energy and this won't be diluted as the universe expands further because this is a property of space itself and more space would mean more of this energy-of-space. Consequently, this will accelerate the expansion, even more, unfortunately, no one understands why this constant should even be there, or why should it have exactly the right value to cause the observed acceleration.
Another explanation comes from the quantum theory of matter which proposed the existence of "virtual" particles in space which continually form and then disappear. But when the energy due to these particles was calculated it came out to be 10120 times too big- wrong by a lot! So the mystery continues…
Another explanation for this dark energy is that it is a new kind of dynamical energy fluid that fills space but whose effect on expansion is opposite to that of matter and normal energy. Some theorists have named this "quintessence" after the fifth element of Greek philosophers. But if quintessence is the answer we still don't know what it's like, or what it interacts with, or why it exists. So again, the mystery continues…
The last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way normal matter in galaxies behaved. So is there a need for a new theory of gravity? If so, then what kind of theory would it be? It must describe the motion of bodies in the Solar System and provide us with a different prediction of the universe that we need. There are candidate theories but none are compelling, so the mystery continues…
The thing we need to conclude, to decide whether the dark energy is a spacious property, a new dynamic energy-fluid or we need a new theory of gravity is more data-better data.
According to the recent research that was published on April 5th, 2021, scientists have proposed a new model in which dark energy has been replaced with Dark matter with a kind of a magnetic force that accounts for the expansion, but more on that later!
- By Muhammad Umair Ansari
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