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Lets have a little competition, who can best explain how a 13.7 billion year old universe can be 156 billion light years in diameter. I kind of get it, but can't explain it clearly, so please help me here.

This is my stab at it:
Imagine the universe just a million years after it was born. A batch of light travels for a year, covering one light-year. At that time, the universe was about 1,000 times smaller than it is today. Thus, that one light-year has now stretched to become 1,000 light-years.

All the pieces add up to 78 billion-light-years. The light has not traveled that far, but the starting point of a photon reaching us today after traveling for 13.7 billion years is now 78 billion light-years away. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.

I was explaining it to our engineering director and he finds the explanation completely lame, I don't know how to make it clearer.

Novelty 8 Apr 12

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@Novelty , here's my stab at this question:

Due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light. The same is true for every location in the Universe relative to any other location in the Universe that is sufficiently far away. The relationship between the size of the Universe, the age of the Universe, and the speed of light in the Universe is simply not 1 to 1. That is, a universe that is 13.7 billion years old need not be 27.4 billion light years in diameter. Since the universe has and is expanding faster than the speed of light, it can be, and is, larger than the speed of light multiplied by its age. []

What do you think? Does that explanation help?

More info on Hubble's Law: []
More info on accelerating expansion:


Just to throw a virtual dog into the fight:

I’m not a physicist, not even a scientist of any stripe, but I have a great interest in this subject, and can follow the discussion, most of which I agree with as far as this thread is concerned.

What I’m chucking in is this; we know that light is affected by gravity, it’s path is bent by stars. The centre of our galaxy, probably all of them, is incredibly dense with stars. Any planets around those very close to the centre are probably in perpetual ‘daylight’ from all directions. This density increase is gradual, increasing as you move to the centre. And on the way are seriously massive stars, quite possibly some far bigger than the biggest we currently have knowledge of, plus black holes scattered hither and yon.

So as we get closer to the centre, increasingly strong gravity is affecting light’s direction constantly. Right at the centre is a super-massive black hole. Light cannot escape a black hole, and this one will have an enormously large event horizon.

What if that gravity field, outside the event horizon, coupled with all the other gravity fields rubbing shoulders with each other, was strong enough to slow light down? What if light travelled through the centre of our galaxy at a lower speed than we experience, gradually increasing in velocity the further it got from the centre? And keeps increasing as it travels through the more sparsely populated parts of the galaxy (e.g: where we are)? And increases further still as it starts to leave any major gravitational influence at all? And increases again as it encounters the gravity of another galaxy (I can already see the flaw there)? Perhaps being slowed again as it confronts the increasingly dense gravity fields of that galaxy (a possible counter to the flaw I noticed)?

I’m fully aware that the math based on our agreement of light’s speed works out. But it is possible that it only works out because of that assumed constant. I should say here that I’m less of a mathematician than I am a scientist.

Please go easy on me - I’m an interested and curious lay person where astrophysics, amongst many others, is concerned.

What say ye?


Practical demonstration. Get a bit of elastic, or cut a rubber band. Mark a point on it. Stretch it by a fixed amount - an inch, say. Mark another point an inch closer to to the end. Stretch it another inch. Mark another point an inch closer. Keep going until the concept that distance traveled in a medium that is itself expanding leads to weird shit (technical term) gets into your intended victim's head.


Can I see that far? No.
Can I smell that far? No.
Can I walk that far? No.
Can I run that far? No.
Can I fly that far? No.
Is it going to make me happy? Healthy? Fulfilled? . . . no?

Well then, why should I concern myself with the issue?

I don’t think your concern was asked for. The question was undoubtedly directed at no-one in particular, but hopefully at those who were interested and could help with an explanation. Your public lack of interest is irrelevant to all but you.

@KevinTwining lol. Just as my reply has evoked a response, that I have no problem with - it is like my comment an opinion. Opinions however are like fallen leaves sometimes the wind will lift them and blow them around, other times they will be firmly affixed to whatever is below them. I must say that your little expose of light and gravity does compound Novelty's original query of novelty and did momentarily excite my brain into thinking about your writing. I will therefore gold star this thread and hopefully have more scientifically or mathematically minded experts than I explain in simple terms that a dunderhead like me can not only understand but be motivated by to follow the thread. So I thank you for your two responses on this thread.


A light year is not a measure of distance but of speed. 186k miles per second. It's like asking which is bigger, a camel or an explosion.

@Novelty Oh, I know nothing about astrophysics. The speed of light, that's all I got.

No, it is a distance measure. Specifically it is the distance light travels in a year.

A light year is a measure of distance; it’s the distance covered by light in one earth year.


My bet is that the engineering director is critical of the Big Bang theory . . . he does not buy into it . . . . and I am right there with him. When they put up the next space telescope, they will have to adjust the figures all over again . . . . it is kind of like how the Catholic church kept claiming that the earth was the center of the universe until 350 years later in the 1990's they finally admitted it . . . . .
Kind of like the flat earthers today, and Ferdinand Magellan . . . "The church says the earth is flat, but I know that it is round, for I have seen the shadow on the moon, and I have more faith in a shadow than in the church."
After all, 13.7 billion is only 17.65% of 78 billion . . . . that is some pretty shitty odds if you ask me.

THHA Level 7 Apr 12, 2019

The universe is getting bigger as it gets older, we're expanding in time and all spatial dimensions. We can see all the older universe but not everything that is happening across the hypersphere. all the other stars who also are looking back to the big bang( think of them as being simultaneously distributed around giant expanding globe with us, they are their "now" and will be far in our past when we can see them.

Yeah, get astrochuck, mine is oldschool Einstein and my best attempt to visualize a fourth dimension


THis sounds like one for @TheAstroChuck


As I understand it, the speed of light has changed since the bar was opened. Ditto the laws of physics changed. But the current limit is to do with the speed of light so it could actually be bigger.

@Novelty here is a simple overview, deeper analysis can easily be found. []


What would that look like if it were put into ( 3.14515926535) Pi?

@Novelty it would be a huge geometric circumference regardless

Thought I was going somewhere with that one a little high on shatter.


I'm not an astrophysicist, nor do I play one on TV, but at the risk of looking stupid I'll take a stab at this. I think your answer makes sense, so I’m just rewording it slightly.

If I understand your engineering director's question: if we assume the universe started at a single point 13.8 billion years ago, then why isn't it just 27.6 billion light years across… the speed of light times two?

It’s a very logical question with a slightly complicated answer. His question doesn't take into account the expansion of spacetime. Light does travels through space at the speed of light, but that space is not static, that is to say it's expanding... so the distance traveled is longer than simply the speed of light.

This is basically the same thing you said, so not sure how much I’ve helped here. The creation rate of new space/time is not bounded by relativity, if that helps. To answer how big the universe really is, and what shape it is well beyond my pay grade.

Hope that helps. I'm sure others more expert than I can elaborate and correct me on anything I said wrong. Have a great weekend!


The Universe itself has expanded since the beginning. Everything in the Universe has moved apart from everything else. In the balloon example mentioned below, picture the Universe as a balloon being blown up. Things inside the balloon expand along with it. There's no more Universe being formed, but what is there is getting bigger.


Inflation is the answer, the universe got 10^78 times more volume in 10^-32 seconds. Crazy, but it has passed all the test so far. Inflationary expansion was way faster than the speed of light! Special relativity says that information cannot move faster than the speed of light, but general relativity says things can move faster than the speed of light as long as no information is transmitted, basically the space between the light expanded, so the light was still moving at the speed of light, just the space around it was growing in dimension faster than the light was traversing it. In fact parts of the universe are likely still inflating. General relativity has also past all the tests, including very weird effects like frame dragging! Inflation and general relativity cover some fairly heavy math, more than can be easily shown here.

Inflation is a powerful force, but it can't beat compound interest.


Maybe I'm not right, but I think the known diameter is 15.6 billion light years. It still represents the same problem. Your explanation, in my opinion, is valid. As the universe expends it carries the time-space fabric, hence the result is a virtual higher than light speed expansion.

zesty Level 7 Apr 12, 2019

@Novelty Yes, I was wrong.


Use the balloon example..


I've no idea about this.. I can't help here?

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