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How many supernovas are going off (visible in principle) right now in the Universe?

Brandon Enright
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JMorehouse
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  • I think that there are something like $10^{11}$ galaxies in the known 'verse, if that helps. – JMorehouse Jun 01 '13 at 21:25
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    Well, you may not want a discussion of simultaneity, but then the only a priori well defined meaning of "now" is "along our past light cone" - i.e. currently visible to us (or would be if they weren't too dim). If you want some other meaning you'd have to say so and define it. –  Jun 01 '13 at 21:34
  • I'm pretty sure they mean number of supernovas that are occurring right now in our reference frame when you factor in the speed of light. So e.g. most of them we won't be able to observe for billions of years. – Izzhov Jun 01 '13 at 21:50
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    @Izzhov But then you need to know how big the universe is, e.g. which horizon to use. There are multiple ways to define this, all leading to different answers. –  Jun 01 '13 at 22:00
  • Per Chris's comment, this only makes sense to ask for the visible universe in our past light cone. Otherwise the answer could very well be infinite (possibly infinite universe). – Brandon Enright Jun 01 '13 at 22:04
  • Honestly just looking for a ballpark estimate of how many could be observed from earth right now. Is is something like $1$, $10^3$, $10^5$, etc? – JMorehouse Jun 01 '13 at 22:29
  • The question explicitly says visible (in two places)... – DilithiumMatrix Jun 02 '13 at 00:24
  • @zhermes I edited visible into the question per jmorehouse's comment so that it wasn't so ambiguous. – Brandon Enright Jun 02 '13 at 01:10

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A typical number that people throw around is roughly a supernova per milky-way-like galaxy per century. A more accurate, larger-scale, average number is something like a few times $10^{-4} \textrm{ Mpc}^{-3} \textrm{ yr}^{-1}$ ([1], [2]). If we take the average sensitive distance to be a couple hundred Mpc, and assume that the average supernovae lasts a couple of weeks, that means that there are roughly 100 SNe visible in the entire sky at any given time (emphasis on the roughly).

How does this compare with survey observations?
Looking at the Palomar Transient Factory, they identify a few hundred supernovae per year, with a field of view of about 10% of the sky - which is consistent with about 100 at any given time.

It would be intersting to look at a more comprehensive list of supernovae and see how those numbers at up.

Now, if you're wondering about the entire universe: using the same numbers and something like $10^{12}$ galaxies, then something like a few supernovae go off every second.

DilithiumMatrix
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As the comments question, it would depend if you meant the entire universe - which the answer could well be as Brandon Enright states - infinite.

But, in terms of what is visible to us, have a read of this rsponse by astronomer Phil Plait and the bottom of this NASA page, initially there was a single supernova visible every century or so, until the invention and refinement in observational techniques and equipment, now dozens are noted each year.

Hope this helps