Expansion of the Universe - Is there a limit?

What is our Universe expanding into?

Our universe is about 13.8 billion years old. Within this stretch of time since the Big Bang, matter (or energy) has taken up all sorts of forms it can take up, in the shape of stars, gigantic galaxies, gas clouds, planets, and ourselves..!! 

But that's not the only thing that happened - the universe expanded; and it still is. A few years ago NASA and the ESA found that this expansion of our universe is accelerating at an ever-increasing rate - accelerating to such a level that some of the farthest galaxies can never ever be observed by us, only because their light is never going to reach us. This means that the universe is expanding at such a rate that not even the speed of light can keep up. 

In other words, our universe is expanding faster than the speed of light!

This is kind of an absurd statement to make since we know from Einstein's Special Relativity that nothing can travel faster than the speed of light. But how can some galaxies which are billions of light-years from us, receding away from us faster than the speed of light? There is one accurate answer to it: the galaxies aren't the only objects receding away, but space itself is!! 

Okay, maybe we're moving way too fast with this whole expansion thing, but if we think about how scientists initially figured out that our universe was expanding, we might get some clarity. 

In the late 1920s, an American astronomer Edwin Hubble, who measured the redshifts of some galaxies, and proved that the universe was expanding (in fact, one of the largest space telescopes that are specially designed for observing deep space, the famous Hubble Space Telescope, was named in honour of Edwin Hubble). Scientists to this day, analyze distance and the light coming from a star or a galaxy by studying its redshift. 

What is Redshift?

The phenomenon of redshifting occurs when the wavelength of the light from the source, elongates, and as a result its wavelength increases and shifts to the red end of the spectrum. This normally occurs for distant stars and galaxies which are getting farther and farther away from us due to the universe's expansion, that the light emitted from them gets stretched and elongated when they reach us because the source galaxy or star continues to move away further. 

When this happens, there is a decrease in the frequency of the light, since wavelength and frequency are inversely proportional to each other. But other than that, there won't be any change in the speed of light. The phenomenon of light being redshifted causes us to view that particular source galaxy to appear redder than it originally is. 

The redshift of a Nebula

There are various types of redshifts, like gravitational redshift, which occurs near a very massive object like a black hole. The gravitational force stretches the light around it and it gets redshifted. Another kind of redshift is what we call the cosmological redshift, which is related to the expansion of our universe. The key point behind the mechanism of redshift is that it happens because spacetime itself is expanding. To be more clear, light is getting elongated because the "fabric" of space containing the light, is expanding further and further away. 

Expanding Space

The easiest way to understand this concept is the balloon analogy, which was firts put forth by Arthur Eddington in 1993. Imagine a balloon (before inflation) and imagine that it has a line drawn on its surface with a marker. Now, imagine that a person fills the balloon with air and it starts expanding. As it expands, the line marked on the surface of the balloon is elongated and stretched. The line will continue to elongate as long as the balloon expands. 

But we know for a fact that the line stretched because the surface itself expanded, leaving the line no choice but to expand along the surface. 

This is exactly how a light wave gets elongated as an aftereffect of the expansion of space. The surface of the expanding balloon can be thought of as the expanding universe. Again, if we mark little dots on the balloon before inflation, those dots will appear farther away from each other after the inflation process. 

Another important point from this analogy is if we observe the whole inflation process of the balloon carefully, we find that it doesn't have a centre. The balloon is not expanding from a specific point (like a centre), but it's expanding from every single point on its surface, just as if every point on the surface was the centre. This also answers the question of whether the universe has a centre from which, everything is expanding farther away. The universe has no specific centre, instead, every point on the spacetime acts as the centre of the universe, and everything recedes away from each other.

Now to our earlier question, how can we say that some of the farthest galaxies are moving away faster than the speed of light when we know that nothing can surpass the speed of light? One of the main conclusions in this topic is that space is expanding. Einstein's Relativity postulates, which includes the speed of light law, is only applicable to local systems, and more importantly, situations where space is unchanging. But in this case, space itself is moving, along with all the energy and matter contained within. And because space is expanding from every single point on it, the farther an object is, the faster it seems to move away from us. This is also called the Hubble law. This is why the farthest galaxies seem to move away faster than the speed of light when they actually don't. 

Another mind-boggling consequence of this is that we receive light from far-flung galaxies that are about 38 billion years away from us when actually our universe is just 13.8 billion years old! This is because when the light initially left the source galaxy after the first 600 million years after the Big Bang, it had to travel all the way through the expanding space, thus increasing its distance travelled. Just because the light travelled about 38 billion light-years doesn't mean that that galaxy was actually 38 billion light-years away. It just shows that space expanded so much during the travelling period of light that it took 38 billion years to finally reach us!! But that's not the weird part - the image that we'll see of the galaxy, that finally reaches us, is not actually its present state; rather we see the image of the galaxy when light initially left it, i.e, as early as about 600 million years aright after the Big Bang!! 

The question is if we're ever going to see the present state of that galaxy? No, and we never will, just because it's receding away from us too fast that we're never going to catch up with even with the speed of light. This part of the universe is literally out of our reach! And as time goes on, more and more of the universe will become out of reach, leaving us in our little Milky Way!! 

We need to understand that this effect of expansion is negligible between local galaxies and systems, because of the gravitational effect, but much more noticeable when we take two far ends in the observable universe. 

CLICK HERE to check out my earlier post on Dark Matter and its properties.