New #AstroPhysicsFactlet
The Universe, beyond our Galaxy, continuously irradiates our planet with any sort of "astrophysical backgrounds", which are the cumulative radiations emitted from all sort of things happening in the Universe - from active galactic nuclei, to exploding stars, to the thermal emission from the Big Bang (CMB) etc.
What is the total power received by our planet at every second?
And how large is the power collected by our telescopes?
If we "weight" the amount of radiation emitted by the Universe as a function of the frequency*, this is the distribution we get. The background is largely dominated by the Cosmic Microwave Background, i.e. the thermal relic radiation from the Big Bang, stretched by spacetime expansion.
All black holes, stars and other processes in the Universe contribute much less.
*=notice this is the observing frequency, and not the emission frequency of each mechanism.
Since the total emission is 99% dominated by the CMB emission, we can just approximate the total extragalactic flux with
F ~ 1000 nW/m^2/std
Now, these are funny units astronomers use, and we need some conversion.
The meaning of it, is to express the power (energy/time) received for each square meter, from a unit of solid angle in the sky (measured in steradians).
nW=1e-9 W=1e-9 J/s
Surface of earth=5e14 m^2
solid angle for the entire sky = 4π=12.6
(A small subtlety here is that each half of our planet's surface can approximately be irradiated only by a half of the celestial sphere, so there is an extra ~0.5 factor to consider.)
So, the total power received by the entire planet is
F_tot ~ F * 1e-9 *S*Ω*0.5 ~ 3.15e9 W
3 billion watts! that's apparently a lot.
That's about the power of 1000 nuclear plant!
However, our Sun produces about a hundred million times more at every second!
The "solar constant" is indeed the amount of radiation emitted by the sun, 1.35 kW/m^2.
Integrated for ~half of the surface of our planet, it gives
F_sun ~ 2.5e17 W
which is ~800 000 000 larger than the total extragalactic flux, integrated over all wavelengths
Now, final question:
how much of the extragalactic radiation has been so far collected by our telescopes?
This is much more difficult to estimate, because:
- most of the extragalactic photons are from the CMB, typically observed by telescopes with ~m^2 size.
- the largest telescopes on Earth are instead in the radio domain, in which we have about a hundreds of telescopes with typical ~100 m^2 sizes - https://en.wikipedia.org/wiki/List_of_radio_telescopes
However, the radio flux there is ~1e-6 lower than the CMB one!
@franco_vazza
I'm confused, are you saying optical telescopes observe the CMB? Or do you mean things like COBE and Plank when you say ~ m**2 areas?
@tobychev I meant (very qualitatively) the second!