how do scientists confirm things like the universe is expanding rather than contracting?
In: Physics
Red shift, light waves stretch out as the relative velocity between two objects increases, that stretching out causes the light to go towards the red end of the visible spectrum.
The basic fact of expansion is fairly easily measured.
We have a great deal of confidence how most stars like our sun works. The emission of light from these stars have a “frequency pattern” in them which indicates things like hydrogen. So this pattern is purely dependent on the element (say hydrogen) and is “fixed”.
By observing the light from stars from far away, we can also detect this pattern meaning it confirms that it is almost certainly hydrogen. However this pattern is “shifted” by a certain amount. This shift tells us that the source of this light is either moving away or towards us and also gives an estimate of how fast they are moving away or towards us.
It happens that a general pattern emerges when looking at light from many different galaxies in many different directions. The further away we believe the galaxy to be, the faster it appears to be moving away from us (in general). This most consistent and simple explanation of this is an expanding universe.
There are a few different bits of evidence. Hope I get them reasonably correct,
Firstly light from a distant object that is moving away from you looks more red. Because the wave length is being stretched. The light from distant galaxies is observed to have shifted red from the expansion of space in between us. The farther they are the redder the light because there has been more time to expand.
Edit: if the universe were contracting then the light would be shifted blue.
Secondly it was predicted that if the universe used to be hotter and denser , then there would be some left over energy that would have cooled and ‘stretched’ if the universe had expanded. And they have found that background energy fitting the prediction.
Thirdly different types of elements like hydrogen and helium need different conditions to be formed. And the balance of types of elements we have detected also fits a hotter denser period followed quickly by an expansion and cooling.
You know how a fire truck that’s coming towards you is slightly higher pitched than when it’s moving away from you?
Light works the same way. The two ends though, instead of being higher and lower pitched, are blue and red (Atleast from what we can see with your eyes). If light coming towards you/ you are moving toward it, it’ll appear slightly more blue than its true color. Vice Versa, things moving away from you will be slightly shifted red.
When we look out into the universe, almost everything is shifted red no matter which way we look. If everything is moving away from us in all directions the only logical explanation is that the universe is slowly drifting away from you, just like all the women in your life.
The gist of it is that they first make observations, and form a hypothesis.
Based on more observations they develop a theory that fits with the observations and (crucially) that can be used to predict development of a phenomenon/value/attribute.
If the theories explanation and prediction works, it’s accepted until someone makes different theory that works *better*.
Ideally, scientists would like to confirm or deny hypothesis through setting up experiments. Having a high level of control over experiments allows them to sus out what details are import and what are irrelevant.
This isn’t always possible. It’s not like scientists can set up orbiting stars in their lab to confirm or deny the existence of gravitational waves. The next best thing is for scientist to collect data, make a hypothesis that fits that data, then look to see if new observations also fit that hypothesis. It would be strictly better if scientist had god like powers to set up their own test universes, but they can still learn things just from observing what’s already going on.
It’s called red shift and blue shift. The easiest was to explain it is the sound difference when a cop car is coming towards you vs away from you the siren gets higher pitched and speed up as the car is lvokg towards you, that’s called blue shift. When the cop car is moving away from you, the sound lowers in pitch and gets slower. That’s red shift.
We look at the light from stars and galaxies and look for either red shift or blue shift. If it’s red shift, they’re getting farther away and the universe is expanding. If they see blue shift, the universe is contracting because those stars and galaxies are getting close to us.
They don’t. It’s a theory at best. Imagine an ant looking into a coke bottle and watching the clouds move, then deciding the world is expanding. There are too many variables for us to use something like red shift to identify expansion. An object moving away from us is not synonymous with expansion.
If humanity makes it, they might know if the universe is expanding in a couple thousand years. Right now, we really don’t know if the movement is related to orbits of these celestial bodies or some other wave manipulating effect in space.
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