# Question about the expansion of the universe

#### mazen_zone

hey,
according to the big bang theory the universe is expanding, thus stars and planets are getting away from each other, but as i know the distance between the earth and the sun is just the suitable distance to keep us alive, and that if the orbit of the earth increased by 3mm all the water on the planet will freeze, and if it's decreased by 3mm all the water will boil,
so how can this distance remain constant while the universe is expanding?

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#### mathman

according to the big bang theory the universe is expanding,
true
thus stars and planets are getting away from each other
false

Space betwen galaxies is increasing, but within galaxies and solar systems, gravity is keeping objects in orbit.

#### marcus

Gold Member
Dearly Missed
if the orbit of the earth increased by 3mm all the water on the planet will freeze, and if it's decreased by 3mm all the water will boil,
so how can this distance remain constant while the universe is expanding?
...
Mathman is right. Big bang cosmology does not say that earths distance from sun is increasing.

It doesn't even say that the size of the Milkyway Galaxy (that the sun belongs to) is increasing. It doesn't say that distances to the galaxies in our local group are increasing.

The Hubble law is a pattern of expansion of much larger distances. The math is simple, but it is math, not words. Some person or website gave you the wrong idea.

I'm curious what you mean by "3 mm". Do you mean 3 millimeters or do you mean 3 million miles, or 3 megameters or what? I would only write 3 mm if I meant 3 millimeters.

#### mazen_zone

I'm curious what you mean by "3 mm". Do you mean 3 millimeters
yes that's what i meant, 3 millimeters, hope this is not wrong too :D

#### marcus

Gold Member
Dearly Missed
yes that's what i meant, 3 millimeters, hope this is not wrong too :D

You are cordially welcome! And you may be relieved to know that we would notice any difference even if the earth were a thousand miles farther from the sun, of a thousand miles nearer to it. The comfortable circumstances for life are not balanced on a knife-edge like that.

There is a lot of misinformation at religious websites that people come here with and a lot of plain old sensationalism in pop-sci websites---for whatever reason. It's hard to say why.

I've never encountered the 3mm notion before. But here's a way to think about it.

A planet's equilibrium temperature goes roughly as the reciprocal square root of the distance. What that means is if you change the distance by 2 percent you will change the temp by about 1 percent.

So say you increase the distance to the sun by 2000 miles which is about 2 thousandths of a percent. That would decrease the temperature by about one thousandth of a percent.

That's in terms of absolute temperature, kelvin degrees. The average temp of earth is around 300 kelvin. So one percent would be 3 degrees and a thousandth of would be three thousandths of a degree.

Maybe five thousandths of a degree on the common Fahrenheit scale. Such a tiny change wouldn't be noticed. This is just a rough estimate to get a ballpark idea of the size of the quantities involved.

But this doesn't have anything to do with the Big Bang picture or with the espansion of the universe, because that doesn't cause the distance from earth to sun to increase.

#### mgb_phys

Homework Helper
So say you increase the distance to the sun by 2000 miles which is about 2 thousandths of a percent. That would decrease the temperature by about one thousandth of a percent.

That's in terms of absolute temperature, kelvin degrees. The average temp of earth is around 300 kelvin. So one percent would be 3 degrees and a thousandth of would be three thousandths of a degree.
Another way to think about it is that the Earth diameter is about 13,000km so at noon in London it is 13,000km nearer the sun than Australia - this doesn't seem to have very much effect on the possibility of life in Australia.

#### mazen_zone

There is a lot of misinformation at religious websites that people come here with and a lot of plain old sensationalism in pop-sci websites---for whatever reason. It's hard to say why.
i don't know, but i heard this info from lots of sources including some Atheist scientists like Richard Dawkins....

#### mazen_zone

Another way to think about it is that the Earth diameter is about 13,000km so at noon in London it is 13,000km nearer the sun than Australia - this doesn't seem to have very much effect on the possibility of life in Australia.
actually -according to my info- the thickness of the "safe" path of the earth is approximately equal to the earth diameter, that's why the whole planet is safe during day and night, but a small shift from this path will get the earth to the "non-safe zone" which will cause severe damage,

#### marcus

Gold Member
Dearly Missed
i don't know, but i heard this info from lots of sources including some Atheist scientists like Richard Dawkins....
I'm skeptical that you would have heard that about 3 millimeters from any professional scientist (regardless of his religious affiliation or lack thereof) because it is simply obviously absurd. I doubt Dawkins, for example, ever said anything remotely like that.

What you should do, I think, is get links to your sources, that you can share with us.
Then we can see if the information at that site is accurate (and you are just misinterpreting it and getting the wrong idea) or if the information is inaccurate or misleadingly presented.

#### Janus

Staff Emeritus
Gold Member
actually -according to my info- the thickness of the "safe" path of the earth is approximately equal to the earth diameter, that's why the whole planet is safe during day and night, but a small shift from this path will get the earth to the "non-safe zone" which will cause severe damage,
There is a simple reason why this is not true. The Earth's orbit is not a perfect circle, it is an ellipse. Thus it does not stay a constant distance from the Sun, but moves closer and further over the course of a year. The difference between its closest and furthest distance is a little over 3 million miles. We are closest around the end of December, and furthest in June. IOW, when it is winter in the Northern hemisphere, we are the closest to the Sun. This is 376 times width of the Earth. IOW, we spend most of the year outside of this supposed "safe" path.

In addition, the average distance of the Earth from the Sun changes with time. At present, it is increasing at a rate of about 523 miles per century. This would mean that in 500AD, the average distance of the Earth would have been closer to the Sun than the width of the Earth.

#### DaveC426913

Gold Member
Earth lives in whst's called the Goldilocks Zone - not too hot and not too cold to sustain life. It is thought to extend from .97 to 1.36 AUs - that's about 36 million miles.

As for expansion of the universe, here's a really rough analogy: Two pennies are glued together with crazy glue, then are attached to a balloon. The balloon is then inflated. The balloon's expansion is far too weak to tear the pennies apart. They remain stuck together. So it is with the Earth and Sun and all things up to galaxy clusters.

#### mazen_zone

well, perhaps i understood this wrong (the 3mm part),

but i read a lot that a shift in the earth path will cause devastation, here's a video for ex. of Richard Dawkins speaks about it:

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#### scibuff

The 3mm part is obviously non-sense. Earth's distance from the Sun changes throughout the year by about 5 million km (from 152 mil km at aphelion in early July to 147 mil km at perihelion in early January). The circumstellar habitable zone within our solar system is from about 0.95 AU to 1.37 AU.

As many already pointed out, the expansion of the universe dominates over gravity only on great distances (millions & billions of light years). In fact, all galaxies in our local group will eventually merge into a single super galaxy while other (non-local) groups will pass beyond the horizon of the observable universe in (a very far far far far far ... ) far away future.

"Question about the expansion of the universe"

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