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Is our universe noisy or silent? Can people hear them? When astronauts are out of the Earth and near an explosion, can they hear its noise.

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    $\begingroup$ Yes, yes there is! Not sound that humans can hear, but since sound is nothing but compressions in gas, and since there is gas in space (albeit very dilute), there is sound in space. Espacially in cosmology, these sound waves play a very significant role. Check out this post at physics.SE $\endgroup$ – pela Aug 8 '16 at 7:27
  • $\begingroup$ Also see this question at physics.SE. $\endgroup$ – David Hammen Aug 8 '16 at 17:13
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There is sound, but you cannot hear it.

1) There is no sound in space: This shows how sound gets fainter in an evacuated glass jar. The same happens with an explosion (or a shout) in space - there is no air to transmit the pressure waves, so no sound.

2) There is sound in space: The interstellar and even intergalactic medium is not completely empty, even though it is much more empty than the best vacuum we can create on earth. Nonetheless, there can be pressure waves in those media.

The bullet cluster
(The bullet cluster, taken from NASA's APOD)

This image shows the mass distribution of two colliding galaxy clusters in blue, and the X-ray gas in red. The galaxies in the clusters have passed through each other without much disturbance, but the intra-cluster gas has collided. The red cone to the right of the centre looks a lot like the sonic boom familiar from aircraft.

For more regular sound waves (repeating pattern as opposed to the one-off of an explosion), you might find those in the interplanetary gas / solar wind of oscillating stars (RR Lyrae or Delta Cephei type stars). I am not sure if anyone has ever measured this.

At any rate, while human ears can pick up sound in the range of ca. $10^2$ to $10^4$ Hz, these sound waves would have a much much lower frequency. Also, the force exerted by these pressure waves on a human ear would be much too small to register.

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As there is no air in space for sound to travel through, you can't hear anything outside of the small bubble you're in, e.g. space suit, space ship.

There is however 'noise' in the electromagnetic spectrum which can be picked up by electronic equipment, with this being an example https://www.youtube.com/watch?v=suSNye-3tVo

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    $\begingroup$ But there's an Air in Space Museum. $\endgroup$ – userLTK Aug 8 '16 at 12:57

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