Understanding Astrophysics: The Hubble Distance and Beyond

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Discussion Overview

The discussion revolves around the concept of the Hubble distance in astrophysics, particularly focusing on the implications of distances exceeding the Hubble distance, the nature of light emitted from distant stars, and the effects of cosmic expansion. Participants explore theoretical aspects, potential misunderstandings, and the relationship between general relativity and cosmological phenomena.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion about the Hubble distance and its implications for observing light from distant stars that recede faster than the speed of light.
  • There is a suggestion that as the universe expands, the Hubble distance increases, potentially allowing light emitted from distant stars to eventually reach us.
  • One participant questions whether observed radiation from distant objects is a purely relativistic effect or if it could also involve quantum phenomena, drawing a parallel to Hawking radiation.
  • Another participant clarifies the distinction between the Hubble sphere and event horizon, noting that objects beyond the Hubble sphere but within the event horizon can emit light that may reach us.
  • Discussion includes the mathematical treatment of light motion in comoving coordinates and the implications of different cosmological models on particle and event horizons.
  • Some participants argue that it is possible to see objects receding faster than light under certain conditions, which has led to confusion in explanations found in textbooks.
  • There is mention of superluminal expansion during inflation and its relation to standard cosmology, with some participants expressing skepticism about the necessity of superluminal expansion to explain the universe's early state.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of the Hubble distance, the nature of light from distant stars, or the necessity of superluminal expansion in cosmological models. Multiple competing views remain, and the discussion is characterized by uncertainty and exploration of various hypotheses.

Contextual Notes

Limitations include varying interpretations of the Hubble distance, the definitions of horizons in cosmology, and the dependence on specific cosmological models. The discussion reflects ongoing debates and unresolved questions in the field of astrophysics.

Who May Find This Useful

This discussion may be of interest to individuals studying cosmology, astrophysics, or those curious about the implications of cosmic expansion and the nature of light in an expanding universe.

  • #31
pervect said:
Very interesting, I must admit - thanks for the explanation. I don't suppose SCC can explain the galactic rotation curves / dark matter problem any better than GR can (or can it?).
No, not yet - its a problem that has been discussed on the GA&C forum.

In SCC there is no need for DE and the total matter density is 22% closure. However SCC BBN requires ~20% baryonic closure density to obtain the correct amount of helium. In other words there is no need for exotic non-baryonic DM either.
However the question that leaves is where is all this baryonic matter?
There is no clear answer to this and as such there is just as much a problem with SCC as there is in GR, which leaves the non-baryonic DM unidentified.
One leading contender though is that the majority of the DM in SCC could be in the form of IMBHs of about 102 - 103 solar masses. These would have formed as the end product of PopIII stars of about the same mass range. A few SMBHs would also form that could become proto-galactic nuclei, some of the IMBHs would be gravitationally bound to them and attract uncondensed gas that formed ordinary stars, planets and ISM. The presence of many PopIII stars going SN at an early stage would ionise the IGM and provide early metallicity. The primordial hydrogen and helium is also seeded with relatively high metallicity in freely coasting BBN and that allows PopIII stars of that mass range to form - metallicity is necessary to radiate the heat away.
Well that's my 'hand waving' scenario! Shoot it down if you want!

Garth
 

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