# Newbie question... The Big Bang & The Observable Universe (and time)

1. Jul 8, 2015

### revv

I am trying to understand some things but I seem to be confused a little...

I was watching a video and it said when you look with hubbles telescope you see the past universe but I seem to have trouble understanding this, does that mean we could see the future if we knew which direction to point the telescope at?

I don't know if this makes any sense... hopefully it does :/

2. Jul 9, 2015

### Drakkith

Staff Emeritus
Well, remember that light doesn't travel from one place to another instantly. It has a velocity of about 180,000 miles per second. So when you look at the Moon, which is about 200,000 miles away, it has taken light a little over 1 second to travel from the Moon to your eye. So whatever happens on the Moon "now" will be seen one second from now. For galaxies that are billions of light years away, we are literally seeing the ancient past because it has taken billions of years for the light to reach us.

It does not.

3. Jul 9, 2015

### Chronos

You appear to assume the universe has a discernable point of origin - it does not.

4. Jul 10, 2015

### revv

Isn't the big bang a point of origin?

5. Jul 10, 2015

### Drakkith

Staff Emeritus
It is not. The big bang theory doesn't even predict a point of origin. All it says is that the universe was originally in a very hot, very dense state and expanded and cooled from there. At no point is the size of the universe ever predicted to be zero.

6. Jul 13, 2015

### revv

Lets say you where to send space ships in EVERY direction from earth into outer space... would they all be considered to be in the past or would some be going into the future?

If not what exactly would happen if we where to do just that?

Sorry if this doesn't make sense im just really trying to understand but I have trouble...

7. Jul 13, 2015

### Chronos

The departing space ships would suffer from time lag compared to time measured on earth due to the finite speed of light. So, you could say they will all journey into our future past

8. Jul 13, 2015

### Bandersnatch

Consider an analogy:

The universe is represented by some large medieval or ancient country, say the Roman empire at its peak. Instead of light signals we have couriers travelling by horse with missives.

Say, there is a system, where each province of the empire sends a courier each day with the most up-to-date news about local state of affairs to the capital.
The couriers need several days to get to the destination - those from nearby as little as one day, those from farther reaches many days.

So, assuming this has been going on long enough for at least one courier from each province to get there, each day, an official (observer) in Rome receives a courier from every province. All couriers carry news that are outdated at the time of reception. The report from a one-day-away province is one day old, the report from a 9-day-away province is 9 days old.

The scenario in your question is analogous with the official sending back some information to all provinces, and order to muster troops for example.
Some couriers will start their return journey on more outdated information than others (maybe the province has been overrun by barbarians in the five or so days it took the latest courier to get to Rome), those also will get there later than others, and the information they eventually send back will be also delayed more.

It's all in the delay caused by signal travel time, but nobody is doing any time travelling here, apart from the mundane sense of travelling into the future one second at a time like we all do.

9. Jul 13, 2015

### revv

If I understand correctly "Local state of affairs" would be the matter in the universe?

10. Jul 13, 2015

### Bandersnatch

The state of that matter. The courier is analogous to the light beam carrying information about that state, i.e. how the province/galaxy looked at the time the courier/light was leaving/was being emitted.

When you receive a courier, you only get to know how about the state in the province at the time he was leaving. When you receive (see) light from a distant galaxy, you only get to know what it tells you about the state the galaxy was when it was leaving.

11. Jul 13, 2015

I thought this would be a good thread to jump into because of the (and time) bit in the topic.
I have always been trouble by the hyperinflation part of the big bang, something conjured up to explain the expansion faster than it should have.
Could it be that at the time of the "Big bang" before any laws of the universe existed, time was not a fixed law, and that time was only "fixed" like all the other laws after the initial expansion. Therefore the initial expansion could have taken its time, so to speak.

12. Jul 13, 2015

### Staff: Mentor

Time is not a "law"; it's a coordinate (or a parameter along timelike worldlines). The inflationary scenario includes the time coordinate/parameter; so when cosmologists talk about the era of inflation taking only $10^{-32}$ seconds (or whatever the time is), that's not adjustable, it's part of the model.

13. Jul 14, 2015

but couldn't 10−32 seconds be meaningless, I mean if time, was stretched at the BB.

14. Jul 14, 2015

### Drakkith

Staff Emeritus
It only matters if there was a difference in the rate of time between different parts of the universe. If the change in time is the same everywhere, then there's no way to measure it and it has no real meaning.

15. Jul 15, 2015

### Finny

As noted, that's because light takes time to get here.

It also means that light carrying the information from an earlier time is coming towards us, no matter which way we look. Light comes TOWARDS us for every direction. If light were only passing us by, we could see nothing in some directions...where the light is receding from us.

That's a huge clue that the origin of that light was not a point in space;Not like a light bulb, for example. Like looking away from the direction of the light bulb...then you can't 'see' it.

16. Jul 18, 2015

### srs101

Another way to put this is that, while all of the universe is basically at the same point in time but because information in the form of light takes time to arrive the observer "you" don't know what happened until the information arrives and because this delay is greater at farther distances the older the information is when it arrives, It is the same principle as the lightning-thunder delay in the way that the sound of the thunder travels "slowly" thus the sound doesn't arrive for several seconds.