Light Year: distance = time?

[Kiefer]

Hello forum members!

 

I read an article the other day concerning a cosmic event that took place 260 million light years away. The article also suggested that this particular event took place 260 million years ago.

 

Even though I understand the concept that, for example, the light of the sun that we see right now is from ~8 minutes ago (given that the sun is 8 light minutes away from the earth), I felt that the article was not taking into consideration that the universe is expanding; and that the event may have occurred later, but only reached us now, given the compounding effect of the universe's expansion on the distance of that body from the earth.

 

Would anyone care to weigh in on that and tell me if I'm completely off here? Could it be that the light from a cosmic event that is 260 million light years away may have occurred much later, but only reached us now because of the universe expansion effect?

 

Thank you!

 

PS - sorry, I don't have any physics background.

[Sensei]

Distance = speed * time

so if speed = c

 

d=c*t

 

Observe what happens to units!

 

We can reverse equation and get

 

t=d/c

 

Distance to Sun is ~150 mln km (150 bln m), c is constant 299792458 m/s

so t = 150*10^9/299792458 = 500 seconds = 8 minutes 20 seconds.

 

260 mln years is very short period of time, less than 2% of 13.6 bln years.

So expansion in that time also is not so much significant.

[imatfaal]

In addition to Strange's answer (which is correct) you should note that 260 Mlyr is also not that far! The milky way is part of the Laniakea Supercluster which is 560 Mlyr across. Superclusters are the largest catagorisation of universal structure and are basically defined as the congregation of those galaxies and clusters that are gravitationally bound to each other; ie the net acceleration of the parts is towards the centre of mass. Within superclusters (to a large extent) you can ignore universal expansion - gravity between the components is strong enough to keep the supercluster together.

 

Expansion happens in the gaps between gravitationally bound superclusters almost by definition; if it is not gravitationally bound and is subject to background expansion then it isn't a supercluster.

[Strange]

In addition to Strange's answer (which is correct) you should note that 260 Mlyr is also not that far!

 

Credit to Sensei for his correct answer!

 

I was about to point out that IF the event were subject to cosmological expansion then then it would mean that the event happened slightly earlier (and closer) than reported value (depending how the 260 M ly figure is defined). I *think* (very rough calculation) it would make a difference of about 2 million light years, which is probably similar to the error bounds on the distance measurement.

[imatfaal]

 

Credit to Sensei for his correct answer!.

...

 

Indeed - dunno what was going on...

[Mordred]

Lol