The edge of the universe?

[Watcher]

I saw the news story about Hubble seeing a galaxy 13.2bn light years away (http://tinyurl.com/4gxrk2b). If the universe is 13.7bn years old then presumably the edge of the universe, as created by the big bang, must be 500m light years beyond that galaxy? IF we can see such a galaxy, albeit as a fuzzy dot, with Hubble now a good few years old, then it can't be long before technology advances sufficiently to be able to comprehend what is beyond the edge of the universe.

[acey]

If the universe is 13.7bn years old then presumably the edge of the universe, as created by the big bang, must be 500m light years beyond that galaxy?

Actually no. Because of the expansion of space, light has to travel extra distance to reach us, and the upshot is that the observable universe has a radius much larger than 13.7 billion light years - more like 47 billion.

Observable universe - Wikipedia, the free encyclopedia

The furthest (=earliest) light we can see is the cosmic microwave background, which comes at us from all directions having started out about 400,000 years after the Big Bang. No way we can ever see light produced earlier than that, though astronomers hope to see other relics such as gravitational waves.

Cosmic microwave background radiation - Wikipedia, the free encyclopedia

[Watcher]

You clearly have a better grasp on these things than me. I get your point about the light taking longer to reach us as the universe kept on expanding.

I can't see, however, how the radius of the universe can be greater than 13.7bn light years away from it's centre as nothing travels faster than the speed of light and the universe began it's expansion (from it's centre point, whereever that it) 13.7bn years ago.

[Stephen]

When astronomers say that the universe is expanding, they’re talking about the rather abstract concept of space-time. Basically, space-time is the three physical dimensions of our existence-length, breadth and depth-combined with the additional dimension of time; think of it as a wire grid that connects every part of the universe to every other part. When we say an object has motion, we’re referring to its change in position relative to the space-time grid. The speed of light is only a constraint for objects that exist within space-time, not for space-time itself.

Source : How could the universe expand faster than the speed of light? That seems impossible! Scienceline

[Watcher]

That's a great explanation, thanks. I love this stuff.

[ollypenrice]

The other thing is that in the BB model the universe doesn't have an edge. It has an observable horizon, beyond which light coming our way will never get here. But, like the surface of a sphere, it is without any point that is different from any other. If you are on the surface of a sphere you can walk all day but never get to an edge.

Olly

[sammy1404]

It is more than 47 billion lt years i believe. I heard that at the moment the diameter its about 156 billion lt years.

There is an edge to the universe but it is constantly moving because of the expansion of space time. And no its not 500 million years more. 13.7 billion years ago was when the expansion began, this was calculated firstly using Hubbles graph of veloctity/distance of observed galaxies. He found that as you increase the distance, you increase the velocity to the velocity of the galaxy is directly proportional to the distance. Anyway, within that 13.7 billion years, space time has expanded. Also i think you are getting confused with 13.7 billion YEARS. Just because the universe is approx 13.7 billion years old doesnt mean that the universe is 13.7 billion light years in diameter, they arent related. Within the 13.7 bn yrs the distance that light has to travel has increased due to the expansion of space time.

Sorry i kind of rambled on there a bit lol.

Oh and as for the bit about hubble. The hubble was sent up in 1990 so not a few years lol Also the Hubble will be out of use sometime around 2016 when NASA plan to launch the James Webb space telescope. It will be primarily an infrared telescope. It is much much larger than the hubble with a mirror of diameter 6.5 metres where as the hubble was just 2.1m. It uses a lot more tech including its unfolding system, because it is so large it has to be folded to fit on the spacecraft and then unfold itself in space, it has hi tech shields to keep the mirror cool because infrared is very sensitive to heat and also it is going way out into space, past the moon. It is going to be out at L2, 1.5 million kilometres from the earth. This is to ensure that the telescope will always remain in the Earths shadow thus minimising thermal energy from the sun to intefere with the telescope.

[George Jones]

I saw the news story about Hubble seeing a galaxy 13.2bn light years away (http://tinyurl.com/4gxrk2b). If the universe is 13.7bn years old then presumably the edge of the universe, as created by the big bang, must be 500m light years beyond that galaxy?

If "edge" is replaced by "radius of the observable universe", then, yes, the radius of the observable universe is 13.7 billion light-years.

Actually no. Because of the expansion of space, light has to travel extra distance to reach us, and the upshot is that the observable universe has a radius much larger than 13.7 billion light years - more like 47 billion.

Yes, the radius of the observable universe is 47 billion light-years.

It is more than 47 billion lt years i believe. I heard that at the moment the diameter its about 156 billion lt years.

Yes, the size of universe is at least 78 billion light-years. The popular media have arrived at 156 billion light-years by incorrectly doubling 78 billion light-years.

My above three statements are seemingly contradictory, but, actually, they are all consistent with current cosmological theory and observations. Why? Because space and distance in cosmology are strange things!

If two different, but correct, ways of measuring distance are used to measure the width of a room, then, to the accuracy of the methods, the results will be same. This is not true in cosmology! Because of spacetime curvature, there is no unique correct concept of spatial distance.

Light travels at a certain finite speed, so the image of any object that we see now is formed by light that left the object some number of years ago. The lookback distance of the object is defined to be the same number of light-years. "the radius of the observable universe is 13.7 billion light-years" uses lookback distance.

Another type of distance is proper distance. Here, "proper" is used in the same sense as "property", not in the same sense as "correct". While the definition of lookback distance considers us and an object at two different times, proper distance considers us and an object at the same instant of cosmic time. "the radius of the observable universe is 47 billion light-years" uses proper distance at the time now.

While lookback distance and proper distance are defined differently, they are both correct. Given distance using one definition, it is possible to calculate the distance given by the other definition. For example, the 13.2 billion light-year lookback distance given in the original post corresponds to a proper distance of 31.7 billion light-years.

And there other definitions of cosmic distance. It is important to keep in mind which definition is being used.

Now on to the 78 (156) billion light-year figure. Current cosmological theories indicate that although 4-dimensional spacetime is curved, 3-dimensional space is very close to being flat. For the sake of a shorter explanation, let's assume that space is flat.

Even if space is flat, it might be connected in strange ways, like in the video game Asteroids. In Asteroids, if you fly off the top of the screen, you immediately appear in the same place at the bottom of the screen. Similarly for the sides of the screen. Maybe the universe is connected up in the same way.

A few years ago, some physicists looked at cosmological data and conclude that if the universe is connected like an Asteroids screen, then the width of the Asteroids screen is at least 78 billion light-years. Note: 1) this is something quite different then the size of the observable universe; 2) the popular media took this 78 billion light-year width or diameter incorrectly as a radius and doubled it to get 156 billion light-years.

[acey]

Very nice explanation, George. Where exactly did the 78 billion figure come from? I'm aware of the work by Janna Levin etc on pacman type universes, and constraints placed on them by the cosmic microwave background - is that where the figure comes from?

Andrew

[George Jones]

Very nice explanation, George. Where exactly did the 78 billion figure come from? I'm aware of the work by Janna Levin etc on pacman type universes, and constraints placed on them by the cosmic microwave background - is that where the figure comes from?

Andrew

The number does come from looking at the cosmic microwave back ground. The abstract for the research paper is at

[astro-ph/0310233] Constraining the Topology of the Universe,

and the the link to entire pdf article is at the top right. From the abstract "For a wide class of models, the non-detection rules out the possibility that we live in a universe with topology scale smaller than 24 Gpc"

Levin writes some interesting stuff (for the general public) at

In space, do all roads lead to home? | plus.maths.org.

[acey]

Thanks!

[magic77]

What if the Beetle eats all the raisins!!.....Will the holes that remain be black holes?....That was a brillinat read...Thanks...

[Ianladd]

I was confused at the first post and definitely confused by the end of the thread.. Really interesting way of explaining the expansion of the universe by relating it to Malt Loaf though! Soreen has never seemed more interesting