How far into the Universe can we theoretically reach?

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Discussion Overview

The discussion revolves around the theoretical maximum distance a spaceship could travel from Earth, considering various factors such as speed, the expansion of the universe, and the nature of light signals. Participants explore the implications of traveling at relativistic speeds and the limitations imposed by cosmic expansion on reaching distant galaxies.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant suggests that if a ship travels indefinitely at a high speed, it could theoretically cover unlimited distance over unlimited time.
  • Another participant proposes that the question may be better framed with a time limit, as distance is the product of speed and time.
  • Some participants discuss the concept of redshift and how it relates to the distance of galaxies, indicating that there are galaxies from which signals sent now would never arrive due to cosmic expansion.
  • A participant mentions that if a spaceship behaves like a light signal, it could reach a maximum redshift of about 0.7, corresponding to a distance of approximately 8 billion light years, factoring in the need for a round trip.
  • Another participant estimates that the maximum distance someone could reach without concern for returning is just under 16.5 billion light years, while an explorer intending to return could reach under 8 billion light years.
  • Some argue that there is no limit on how far one can travel, as the separation velocity could increase indefinitely with the universe's expansion.
  • Others clarify that while one can keep moving, there are distances that, due to the accelerating expansion of the universe, one would never be able to reach.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the maximum distance achievable and the implications of cosmic expansion. There is no consensus on a definitive answer, as participants present differing interpretations of the effects of expansion and the nature of travel through space.

Contextual Notes

Participants note that the discussion hinges on assumptions about the universe's expansion rate and the behavior of objects traveling at relativistic speeds. The complexity of these factors leads to varying interpretations of maximum reachable distances.

  • #31
Vick said:
OP asked about furthest distance we can reach using speed of light fraction in a starship setting. This is asked due to the expansion of the universe having an effect on the question at hand.

Therefore maximum speed is c 299,792.458 km/sec
Expansion of universe factor is H0 = 67 km/sec/Mpc (Megaparsec)

Thus c/Ho = 4474.514 Mpc (or 14586.92 light years (ly) or 14.6 billion ly)
This is not the correct answer.
What you have calculated there is the Hubble radius, i.e. the distance at which recession velocity reaches the speed of light. This in and of itself is not a horizon. For example, such radius exists even in expansion models that are not accelerating. At the same time, in those models, it is possible for a signal to reach arbitrarily far, given enough time (cf. 'ant on a rubber rope' exercise, e.g. on Wikipedia).

The limit to the reach of a signal exists only in accelerating models, and is determined by the distance to the cosmic event horizon, which has already been discussed earlier in this thread. At present, this horizon is a good couple billion light years further out than the Hubble radius.

The Hubble radius and the event horizon can coincide, but this only happens in exponential expansion models, which are fully dominated by dark energy (i.e. have no matter or radiation in them). This happens during inflation. It's also what our universe appears to be evolving towards - but only asymptotically so.
 
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  • #32
Ibix said:
"To infinity and beyond" is the catchphrase of Buzz Lightyear from the Toy Story franchise. It's aimed at children under 12. I propose that there's little value in arguing about its scientific validity.
Well you know, in set theory there are quite bizarre notions of infinity.
I am waiting to find the time to read Jech's green book on Set Theory.
 
  • #33
MathematicalPhysicist said:
I am waiting to find the time to read Jech's green book on Set Theory.
I always wonder what people mean when they say that. Where are you going to look? Did you perhaps leave the time in your other pants? Under the bed?

Personally, I believe in making time in my schedule to do things, or not making the time if i decide it's just something I'm not ever really going to do.
 
  • #34
phinds said:
I always wonder what people mean when they say that. Where are you going to look? Did you perhaps leave the time in your other pants? Under the bed?

Personally, I believe in making time in my schedule to do things, or not making the time if i decide it's just something I'm not ever really going to do.
Well, I have this book on my shelf hard copy which I purchased back then, so I intend to read it sometime in the future.

Today, I am reading on off the handbook on QCD of Mueller's.
Anyway, if the problem of time in quantum cosmology is real, then time is an illusion anyways...
The book by Jech I started reading and finished reading chapter one, but never continued to read.
 
  • #35
MathematicalPhysicist said:
Well, I have this book on my shelf hard copy which I purchased back then, so I intend to read it sometime in the future.

Today, I am reading on off the handbook on QCD of Mueller's.
Anyway, if the problem of time in quantum cosmology is real, then time is an illusion anyways...
The book by Jech I started reading and finished reading chapter one, but never continued to read.
The key is to draw a circle and write “tuit” inside. Cut it out, and you’ve got ”a round tuit”.
 

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