Q At the moment of the big bang, all mass was in a single spot and has been expanding outward from there since. Where is the centre of the expansion from, and wouldn't there be a growing void emanating from that point going forward? asks Kirk Lynn
A As Dr David Mulryne from Queen Mary University of London points out: 'Because light takes a finite time to reach us from distant objects, and because the universe has a finite age since the big bang, we only know about the region of the universe that we can see.' We do, however, have a pretty good model for this region, known as the Friedmann-Lemaître-Robertson-Walker (FLRW) model, he adds. 'In this model there is no centre, rather every point in the universe, on average, looks the same as every other.'
To get a handle on what's going on, conjure up an image in your mind of an enormous, indeed infinite, sheet of paper. 'Every point on the sheet of paper looks like every other,' says Mulryne. 'We could imagine there being dots drawn on the paper on average evenly distributed, representing the galaxies that fill our universe - no matter which galaxy an observer lives in, the universe around them would look roughly same.'
But what happens if we stretch the paper uniformly in all directions? 'If we imagined grid lines on the paper we can imagine the grid spacing growing,' says Mulryne. 'An observer in any one of the galaxies would see all the other galaxies get further away. This is what we observe in the real universe when we look at distant galaxies, but in three spatial dimensions.'
If you could travel backwards in time, however, objects in the universe would be closer together.
'In our analogy this means the grid spacing was smaller, and if this continues into the past until a time when the grid spacing was zero, the area of the sheet would go to zero, and all the dots on the paper would have sat on top of one another,' explains Mulryne. 'We call this time the big bang.'
As the universe expands, everyone sees galaxies moving away. 'Every observer will imply that they were closer to their neighbours in the past, and that there was a time when all their neighbours were on top of them - as if they were the centre of an explosion,' says Mulryne. 'And yet, observers in all the other galaxies would see the same thing, so there was no centre, rather every point is roughly equivalent to every other.'
Q How far from Earth will radio/television/human generated signals be distinguishable from the cosmic background radiation? asks 'D Ata'
A To start, let's imagine you're in space, peering back at our planet. In that case, as Dr Angela Taylor from the University of Oxford explains, whether you can detect man-made transmissions above the background noise of the cosmic microwave background 'will depend on how big a radio telescope you have and what frequency the transmissions are made on'.
Let's assume you have a state-of-the-art radio telescope like the Square Kilometre Array and, optimistically, the receivers don't add any noise - the only noise comes from the CMB. Taylor reveals that the CMB is bright across all radio frequencies so let's work our way across the spectrum, kicking off with AM radio that has frequencies of around 1MHz. 'At this frequency, radio waves don't even escape from the Earth as they are blocked by the ionosphere,' says Taylor.
'FM radio is at a higher frequency of 100MHz and will escape from the Earth. However, at this frequency the background glow from our own galaxy is about 100 times brighter than the CMB and so this will always limit your ability to detect it.'
What about TV? 'TV is around 800MHz and the galactic background is still about as bright as the CMB, even in the least bright parts of the sky. It is only really when we get to frequencies around 1 or 2GHz that the CMB becomes the dominant background.'
While mobile phones operate at frequencies in this range, could we detect them above the CMB? 'There are about 6bn mobile phones on the Earth and they typically produce ~1W of power,' says Taylor. 'Making reasonable assumptions about how many are switched on and can be seen at any one time, if our telescope was to stare at the Earth for a whole day collecting the signals, we could just about detect the presence of mobile phones on Earth above the CMB at a distance of ~4,000AU. This is well outside the solar system, but nowhere near the closest star (Proxima Centuri at 4.2 light years or ~270,000AU).'
Could this play havoc with spacecraft like the Planck observatory that was looking at the CMB? Taylor says not.
'Planck won't be affected by mobile phone signals - it is observing at much higher frequencies and it is a very small telescope,' she says. 'It seems a bit paradoxical, but although Planck is very sensitive to the CMB (which is present everywhere) it is not very good at picking up signals from a single point source like a radio transmitter.'
Q Why is it that divers seem to produce such small splashes when they enter the water? asks Amara
A A small splash depends on how the swimmer enters the water. 'Divers produce big splashes when the water near the surface is forced outward much larger than their shoulders,' says Dr Tadd Truscott, of Brigham Young University.
'For instance, when a person does a 'cannonball' they typically make a much wider cavity than when they dive ... but if they dive with too much rotation as they hit the water then the cavity can be much wider or longer than their shoulder width.'
What happens to this cavity is important. 'When the diver hits the water, they open up a cavity that has a width at the surface but also a depth associated with it (1-4 feet deep),' says Truscott.
It turns out that this cavity is 'storing energy' and the water wants to bounce back. 'To do this, it [the cavity] collapses,' explains Truscott.
'The collapse causes a large jet to form, a singularity we often refer to as a 'worthington jet'. In the case of a falling cannonball, this jet can rise up many meters above the surface. For a human doing a 'cannonball' dive can also cause this large secondary splash.'
But not all dives create such splashes. 'Depending on the orientation of the diver and how large the opening of the cavity is, this secondary jet can come splashing up above the surface or splash into the side of the cavity, rather than coming to the surface,' Truscott explains.
'When the diver executes a really precise dive, the cavity actually collapses on the diver's legs and body, allowing the cavity to close more cleanly.'
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