Editing Faster-than-light (section)

===Cosmic expansion===

According to [[Hubble's law]], the [[expansion of the universe]] causes distant galaxies to appear to recede from us faster than the speed of light. However, the recession speed associated with [[Hubble's law]], defined as the rate of increase in [[comoving and proper distances|proper distance]] per interval of [[cosmological time]], is not a velocity in a relativistic sense. Moreover, in [[general relativity]], velocity is a local notion, and there is not even a unique definition for the relative velocity of a cosmologically distant object.<ref>
{{cite web |last=Wright |first=E. L. |date=12 June 2009 |title=Cosmology Tutorial – Part 2 |url=http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD |access-date=2011-09-26 |work=Ned Wright's Cosmology Tutorial |publisher=[[UCLA]]}}</ref> Faster-than-light cosmological recession speeds are entirely a [[coordinate conditions|coordinate]] effect.

There are many galaxies visible in telescopes with [[redshift]] numbers of 1.4 or higher. All of these have cosmological recession speeds greater than the speed of light. Because the [[Hubble's law#Interpretation|Hubble parameter]] is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.<ref>See the last two paragraphs in {{cite web
|last1=Rothstein |first1=D.
|date=10 September 2003
|title=Is the universe expanding faster than the speed of light?
|url=http://curious.astro.cornell.edu/the-universe/cosmology-and-the-big-bang/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/616-is-the-universe-expanding-faster-than-the-speed-of-light-intermediate
|website=Ask an Astronomer
}}</ref><ref name="ly93">
{{cite web
|last1=Lineweaver |first1=C.
|last2=Davis |first2=T. M.
|date=March 2005
|title=Misconceptions about the Big Bang
|url=http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf |archive-url=https://web.archive.org/web/20060527202701/http://www.mso.anu.edu.au/%7Echarley/papers/LineweaverDavisSciAm.pdf |archive-date=2006-05-27 |url-status=live
|work=[[Scientific American]]
|pages=36–45
|access-date=2008-11-06
}}</ref><ref>
{{cite journal |last1=Davis |first1=T. M. |last2=Lineweaver |first2=C. H. |year=2004 |title=Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe |journal=[[Publications of the Astronomical Society of Australia]] |volume=21 |issue=1 |pages=97–109 |arxiv=astro-ph/0310808 |bibcode=2004PASA...21...97D |doi=10.1071/AS03040 |s2cid=13068122}}</ref>

However, because [[accelerating expansion of the universe|the expansion of the universe is accelerating]], it is projected that most galaxies will eventually cross a type of cosmological [[event horizon]] where any light they emit past that point will never be able to reach us at any time in the infinite future,<ref>
{{cite journal
|last=Loeb |first=A.
|year=2002
|title=The Long-Term Future of Extragalactic Astronomy
|journal=[[Physical Review D]]
|volume=65 |issue=4 |pages=047301
|arxiv=astro-ph/0107568
|bibcode=2002PhRvD..65d7301L
|doi=10.1103/PhysRevD.65.047301
|s2cid=1791226
}}</ref> because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in {{Section link|Comoving and proper distances|Uses of the proper distance}}). The current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signal would never reach us if the event was more than 16 billion light-years away.<ref name="ly93"/>