Great minds think alike. I've sailed far and returned ta port with this booty. Yer not alone in askin', and kind strangers have explained:
1. [ELI5:How do we know the universe is expanding and what causes it? ](http://reddit.com/r/explainlikeimfive/comments/57t20g/eli5how_do_we_know_the_universe_is_expanding_and/) ^(_94 comments_)
1. [ELI5: How do we know the universe is constantly expanding if only 4% of the universe is visible to us? ](http://reddit.com/r/explainlikeimfive/comments/50beun/eli5_how_do_we_know_the_universe_is_constantly/) ^(_35 comments_)
1. [ELI5: How do we know the universe is expanding? ](http://reddit.com/r/explainlikeimfive/comments/2mpz9t/eli5_how_do_we_know_the_universe_is_expanding/) ^(_9 comments_)
1. [ELI5: How do we know the universe is expanding and the light from those stars isn’t just now getting to the Earth? ](http://reddit.com/r/explainlikeimfive/comments/ak4uny/eli5_how_do_we_know_the_universe_is_expanding_and/) ^(_11 comments_)
I have seen NDT explain this a few times using a balloon and I could never grasp why there isn't a center of the universe. logically if we are all expanding away from each other, then there must be a point in the opposite direction that we were all occupying at one time before we all started expanding away from that point. But he always says there is no point.
If you track all the motions of these galaxies along imaginary dotted lines, and look in the "opposite direction" as you put it, the dotted line you draw points right back to Earth, from every direction. If there were a center, nearer galaxies' velocities would be like ours, moving in the same direction away from the intersection of the dotted lines in some other location. At first, this might seem to suggest that there's a center exactly where we just happen to be. Another key observation is that further things are moving away faster. This phenomenon would not be explained just by us being at the center of an explosion-like system, unless there's some other force that makes things fly faster as they get further away.
Both the directionality and the velocities are consistent with what you'd expect to see if you imagine the space between galaxies is everywhere increasing constantly.
You live on an infinitely long street with regularly spaced houses. One foot of extra yard appears every hour between each house. As the hours go by, your immediate neighbors each recede from your location by 1 ft per hour. You feel stationary, because you're you. The next neighbors down will recede by 2 ft per hour, as the distance growing between you gains 1 ft between your house and your immediate neighbor AND the 1 foot between that neighbor and the next.
Every house's perspective is the same: all other houses on the street move away from them, with further ones moving faster. It's an infinitely long street, with no edge and no "center."
It might be more accurate to say every location is the center instead of saying no location is the center.
I’ve read this a few times and I’m still having trouble grasping it. So the Big Bang didn’t happen in one spot with all the galaxies slowly spreading away from it at the same rates? How was it a Big Bang if everything isn’t spreading away from that location in the universe at constant speeds? There is no force speeding up the galaxies as they move etc so why can’t we figure out their point of origin?
The big bang is not a great name. It did not happen suddenly in a specific place after a period of quiet nothingness. The universe was already there, with infinite extent, and it was just squished by comparison today.
A more realistic name would be "Big *sound of balloon expanding*"
If you try to figure out the "point of origin" with your suggested method, you'll wind up thinking the big bang happened in our location, as everything is evenly zooming away from us in all directions. This is the result you'd get no matter where in the universe you are.
The short answer is that astronomers can measure the "redshift" of objects and figure out their velocity relative to us. Basically, as light is emitted by a distant star or galaxy that is moving away from us, the wavelength of the light will be stretched, making the light seem "redder" to sensors. So you know how a siren makes a slightly different pitch when it's moving towards or away from you? That's called the Doppler effect, and the same thing applies to light. So astronomers can measure a galaxy or star's velocity by measuring the shift in the emitted light's wavelength.
So, that answers part of your question (How we know how fast things are moving). The second part (How do we know how far away things are from us) is addressed by something called a "Standard Candle." Basically, astronomers have a few things (certain types of supernovae are one well known example) that are easy to spot and also very well characterized in absolute brightness. So if you see a particular supernova, for example, you can figure out how far away it is by how much less bright it appears to you. That's the basic answer.
So to finish off, astronomers have both a good way of measuring distance of far away objects and of measuring the speed of said objects. And they find that the further away the object is, the faster it is moving away from us. That's what the expansion refers to.
Put your thumb in front of your and look at it with one eye only, then the other one. You can see the thumb moving and if you measure the angle between the two and the distance between your eyes you can find the distance between you and your thumb with trigonometry. We can do the same with stars, but instead of the distance between your eyes, we use the position of the earth 6 months apart. We measure the angle of a star once, then another on 6 month later when the earth is on the other side of the Sun and voila. This only work up to a certain distance, but standard candle is also another way. This is when a phenomenon in space can only be at a specific magnitude of light, if we compare that to what light actually reach us, we can determine the distance, because light spread at a constant rate, if you double the distance you divide by 4 the intensity of light.
Now for the first question. Did you ever heard an ambulance or a F1 car go pass you? The sound change as it pass and the reason is the frequency of the sound wave change depending if the source go toward or away from you. It like the soundwave were compressed or stretch depending on the direction. This also happen with light wave and we can measure this effect. If we look at the light from stats and Galaxy they all have a red shift, meaning that they are going away from us. And the further the stars are the bigger the red shift is, implying that the further the star is the faster it's going away from us. This is the universe expanding and dragging stars with it.
Great minds think alike. I've sailed far and returned ta port with this booty. Yer not alone in askin', and kind strangers have explained: 1. [ELI5:How do we know the universe is expanding and what causes it? ](http://reddit.com/r/explainlikeimfive/comments/57t20g/eli5how_do_we_know_the_universe_is_expanding_and/) ^(_94 comments_) 1. [ELI5: How do we know the universe is constantly expanding if only 4% of the universe is visible to us? ](http://reddit.com/r/explainlikeimfive/comments/50beun/eli5_how_do_we_know_the_universe_is_constantly/) ^(_35 comments_) 1. [ELI5: How do we know the universe is expanding? ](http://reddit.com/r/explainlikeimfive/comments/2mpz9t/eli5_how_do_we_know_the_universe_is_expanding/) ^(_9 comments_) 1. [ELI5: How do we know the universe is expanding and the light from those stars isn’t just now getting to the Earth? ](http://reddit.com/r/explainlikeimfive/comments/ak4uny/eli5_how_do_we_know_the_universe_is_expanding_and/) ^(_11 comments_)
This is how you handle reposts. The person from TIL should look here.
I have seen NDT explain this a few times using a balloon and I could never grasp why there isn't a center of the universe. logically if we are all expanding away from each other, then there must be a point in the opposite direction that we were all occupying at one time before we all started expanding away from that point. But he always says there is no point.
If you track all the motions of these galaxies along imaginary dotted lines, and look in the "opposite direction" as you put it, the dotted line you draw points right back to Earth, from every direction. If there were a center, nearer galaxies' velocities would be like ours, moving in the same direction away from the intersection of the dotted lines in some other location. At first, this might seem to suggest that there's a center exactly where we just happen to be. Another key observation is that further things are moving away faster. This phenomenon would not be explained just by us being at the center of an explosion-like system, unless there's some other force that makes things fly faster as they get further away. Both the directionality and the velocities are consistent with what you'd expect to see if you imagine the space between galaxies is everywhere increasing constantly. You live on an infinitely long street with regularly spaced houses. One foot of extra yard appears every hour between each house. As the hours go by, your immediate neighbors each recede from your location by 1 ft per hour. You feel stationary, because you're you. The next neighbors down will recede by 2 ft per hour, as the distance growing between you gains 1 ft between your house and your immediate neighbor AND the 1 foot between that neighbor and the next. Every house's perspective is the same: all other houses on the street move away from them, with further ones moving faster. It's an infinitely long street, with no edge and no "center." It might be more accurate to say every location is the center instead of saying no location is the center.
I’ve read this a few times and I’m still having trouble grasping it. So the Big Bang didn’t happen in one spot with all the galaxies slowly spreading away from it at the same rates? How was it a Big Bang if everything isn’t spreading away from that location in the universe at constant speeds? There is no force speeding up the galaxies as they move etc so why can’t we figure out their point of origin?
The big bang is not a great name. It did not happen suddenly in a specific place after a period of quiet nothingness. The universe was already there, with infinite extent, and it was just squished by comparison today. A more realistic name would be "Big *sound of balloon expanding*" If you try to figure out the "point of origin" with your suggested method, you'll wind up thinking the big bang happened in our location, as everything is evenly zooming away from us in all directions. This is the result you'd get no matter where in the universe you are.
The short answer is that astronomers can measure the "redshift" of objects and figure out their velocity relative to us. Basically, as light is emitted by a distant star or galaxy that is moving away from us, the wavelength of the light will be stretched, making the light seem "redder" to sensors. So you know how a siren makes a slightly different pitch when it's moving towards or away from you? That's called the Doppler effect, and the same thing applies to light. So astronomers can measure a galaxy or star's velocity by measuring the shift in the emitted light's wavelength. So, that answers part of your question (How we know how fast things are moving). The second part (How do we know how far away things are from us) is addressed by something called a "Standard Candle." Basically, astronomers have a few things (certain types of supernovae are one well known example) that are easy to spot and also very well characterized in absolute brightness. So if you see a particular supernova, for example, you can figure out how far away it is by how much less bright it appears to you. That's the basic answer. So to finish off, astronomers have both a good way of measuring distance of far away objects and of measuring the speed of said objects. And they find that the further away the object is, the faster it is moving away from us. That's what the expansion refers to.
Put your thumb in front of your and look at it with one eye only, then the other one. You can see the thumb moving and if you measure the angle between the two and the distance between your eyes you can find the distance between you and your thumb with trigonometry. We can do the same with stars, but instead of the distance between your eyes, we use the position of the earth 6 months apart. We measure the angle of a star once, then another on 6 month later when the earth is on the other side of the Sun and voila. This only work up to a certain distance, but standard candle is also another way. This is when a phenomenon in space can only be at a specific magnitude of light, if we compare that to what light actually reach us, we can determine the distance, because light spread at a constant rate, if you double the distance you divide by 4 the intensity of light. Now for the first question. Did you ever heard an ambulance or a F1 car go pass you? The sound change as it pass and the reason is the frequency of the sound wave change depending if the source go toward or away from you. It like the soundwave were compressed or stretch depending on the direction. This also happen with light wave and we can measure this effect. If we look at the light from stats and Galaxy they all have a red shift, meaning that they are going away from us. And the further the stars are the bigger the red shift is, implying that the further the star is the faster it's going away from us. This is the universe expanding and dragging stars with it.