Exploring Ancient Universe Proximities: 12,000 BC - 400 BC

[ASTRA225o4]

Hi, if possible I would like to ask a question to experts in this area, it is correct to say that: In ancient times, ranging from around 12,000 BC. until 400 BC, the universe was more compact, and consequently the distances between solar systems were smaller? From what I have been able to understand, as an enthusiast and not as a scholar, the universe is expanding and accelerating that expansion continuously. If so, was there a time when the proximity between some solar systems was less than what we can observe today? To give a stupid example: if we take Sirius A and B which are about 8.6 light years from the earth, is it possible that at a certain time they were visible to the naked eye because they were closer to the earth? Thanks to everyone who will help me understand, I hope I explained myself well.

 

[Ibix]

No. The distances between distant galaxies are growing as you say, but nearby galaxies are bound by their gravity and are not separating. Nor are solar systems moving apart, since they are similarly bound together.

As I recall, the nearest system is actually moving slightly towards us, but not at any significant rate compared to the distance between us.

 

[ASTRA225o4]

Ah I see. Thank you so much for the answer!

 

[lomidrevo]

In ancient times, ranging from around 12,000 BC. until 400 BC

These timescales are nothing whe compared to cosmic timescale. The universe was approximately the same.

To give a stupid example: if we take Sirius A and B which are about 8.6 light years from the earth, is it possible that at a certain time they were visible to the naked eye because they were closer to the earth?

Sirius A is visible to naked eye. We just cannot recognize Sirius B by naked eyes. And more importantly, the expansion of universe doesn't affect bound systems like stellar systems, galaxies or even the galaxy clusters. So the answer is no.

 

[Vanadium 50]

Sirius is moving, but it has nothing to do with the expansion of the universe. It's also moving in the wrong direction - towards Earth at 5 or 6 km/s. In the last 10,000 years it's moved about 2% closer.

 

[Klystron]

Other factors affect viewing dim stellar objects from Earth. Within my own lifetime under clear conditions on dark nights the Milky Way galaxy made a bright swath across the sky above our valley. Air and light pollution now mask all but the brightest stellar objects near cities.

While astronomical distances have not changed appreciably in a few thousand years, civilization has altered viewing.

 

[Keith_McClary]

Sirius is moving, but it has nothing to do with the expansion of the universe.

If Sirius (and we) were moving only with the expansion of the universe, without gravitation, how much would the distance have increased in 10,000 years? (Assume Sirius has negligible mass.)

 

[Vanadium 50]

Zero. As said above, expansion doesn't apply to bound systems.

 

[Bandersnatch]

If Sirius (and we) were moving only with the expansion of the universe, without gravitation, how much would the distance have increased in 10,000 years? (Assume Sirius has negligible mass.)

If these weren't actual stars in a galaxy, but two points of negligible mass in a void, moving with the Hubble flow, then they'd separate by 1/1440000 of the distance between them.

 

[PeroK]

If Sirius (and we) were moving only with the expansion of the universe, without gravitation, how much would the distance have increased in 10,000 years? (Assume Sirius has negligible mass.)

The expansion rate is derived from the Friedmann equation, and uses an average universal mass/energy density. The Friedmann equation cannot be applied to our galaxy, so in no sense is the space in our galaxy expanding.

For example, the behaviour of the galaxy does not change over time. It doesn't matter how large the universal expansion becomes, it will never affect the dynamics of the galaxy itself. Eventually, the universal expansion rate may be huge, but that wouldn't affect bound systems like a galaxy. The equation that governs the overall expansion, based on the average mass/energy density of the universe, simply does not apply to the galaxy.