Physics
If time is relative, how do we know the age of the universe?
Assuming we are not cosmology experts here, I have tried to set the minimal context to answer the question titled here. Let’s get started and I promise we will be there soon.
Why should I believe that the universe has an age? Why can’t it just exist forever and we just happen to be here at this moment, in time?
We have experimentally verified that the universe is rapidly expanding (all galaxies are moving away from each other).
No single medium post can cover the whole Big Bang Theory but what we mean by the age of the universe is the moment when all universe’s matter was condensed to a small point(t ~0) till now.
How and why is time relative?
Not that I need to answer this, thanks to a plethora of sci-fi movies and. Still,
In 1905, Einstein proposed that for all non-accelerating observers, the speed of light is observer-dependent. This immediately implies that space and time become observer-dependent. It took him 10 more years to add acceleration and gravity into this picture. His theories have been experimentally verified to extreme limits on the macro universe and hence, we believe him.
To summarise, time is an observer-dependent concept and we perceive time differently from another observer who is either
(a) in a relative motion to us. OR
(b) in a different gravitational field than us
So, what does it mean “the universe is 13.8 billion years old” ?
- We have established that to measure time, we need an observer. So, who is this observer? Am I talking about an Earth observer?
“Well, it can’t be because the Earth is only 4.5 billion years old.”
Umm, actually yes and no. It is definitely not the earth but the reason is not that the earth didn’t exist for that long. Even if we assume that the Earth got created at the birth of the universe and an observer on Earth has been experiencing constant gravity of 9.8 m/s² ever since, that observer wouldn’t measure the age to be 13.8 billion years.
Who is this observer we conveniently or inconveniently ignore when we talk about the age of the universe?
This is the faint (CMB)cosmic microwave background radiation, filling all space. It is the static sound your radio receives when the antenna is not tuned to a specific frequency.
The universe was extremely hot when it was created and it was expanding. All material was ionized (called plasma) as the temperature wasn’t cool enough for any atoms to form. Any light emitted by plasma would run into the sea of electrons and hence couldn’t escape the bubble.
At about 400,000 years after the formation, the universe had expanded and was cool enough (~3000K) for the first atoms to form.
The last of the light emitted by that plasma material before it was neutralized was able to stream across the space, unchallenged by any free electrons.
This CMB is that light, the earliest possible picture of our universe.
Very cool! This is the picture but where is the observer?
Before we find our observer, just one last revision.
A redshift is an increase in the wavelength of light (redshift means blue light can become red due to stretch in the wavelength). There are many reasons this can happen but we are concerned with the relativistic Doppler effect(when source and observer are moving away from each other).
Similarly, blueshift will happen when the source and observer travel towards each other.
An observer who will perceive the universe, i.e. the CMB without any blueshift or redshift (this is called being isotropic to CMB) is called a comoving observer. Non-comoving observers (due to a relative motion b/w them and the “universe”)will see regions of the sky systematically blue or redshifted.
General Relativity(GR) can choose any arbitrary frame for calculation. However, choosing a comoving observer frame is more natural and easier to work in and this is the frame in which we apply GR to our experimental data to deduce the age of the universe to be 13.8 billion years old.
This is the frame w.r.t which we talk about any distance and age on the astronomical scale (like the diameter of the observable universe, age of our Milky Way, the stars, etc). Yeps, so that’s our special frame.
Thank you for your patience if you reached here.
Out of many of Einstein’s novel ideas inspiring GR, one was the principle of equivalence, the underlying idea being that no frame is special. The principle of equivalence holds true, but it just seems like nature conspired to pick a special frame for itself when our universe popped into existence.
