As I understand it, every point in space is expanding away from every other point faster than the speed of light, and atoms are mostly empty space. So why isn’t our height measured in light years?
Maybe it is.
The rulers are getting longer, too.
The forces holding us together (electromagnetic in the case of our bodies, gravity in the case of planets, stars, and galaxies) are way stronger than whatever is inflating space.
And on a local scale, the inflation is not faster than the speed of light. IIRC that’s only the case for things that are on the opposite side of the universe.
Because the ‘force’ from expanding space is incredibly weak on personal scales, which means that the four fundamental forces are much stronger and keep objects together in a local sense. The electromagnetic forces in your body keep it the same size while space expands, and gravitational forces hold the Earth, the Sun, the Milky Way, and out to some level of clusters together in defiance of the expansion of space. Viewing atoms as ‘empty space’ isn’t really accurate, the space inside of an atom is filled with various fields that interact to keep the components together.
Also, if expansion was strong enough to spread your body out over light years, you’d be as dead as if you manually spread the components of your body out over light years.
Every point in space is not expanding away from every point of space at greater than the speed of light, that’s just a misconception or bad explanation you picked up somewhere.
Right. The correct version is that every point in space can consider itself to be the center of the universe. From that center every point more than 13.8 billion light years away is moving faster than the speed of light and so is no longer visible.
Just for the record, my height is 1.906 x 10^-16 light years.
–Mark
Typical short guy… has to include the .006.
Not only is the speed of expansion not the speed of light, but it isn’t even a speed. It’s a frequency, though it’s most often expressed as a speed divided by a distance. The rate is about 70 kilometers per second per megaparsec: In other words, if two points are a megaparsec apart, then they’re separating from each other at 70 km/s, and if they’re 2 Mpc apart, they’re separating at 140 km/s, and so on. But human-scale distances are far, far less than a megaparsec (which is over 3 million lightyears), so on human scales, the rate of separation is too small to notice.
To put a more exact definition on this, two particles are separated by a distance greater than the Hubble radius, they cannot talk to each other from now on. This distance is currently around 4.1 Gpc but will get shorter over time.
The particle horizon or the greatest distance from which light could have reached the observer by a specific time continues to grow.
The cosmic event horizon is the largest distance from which light emitted now can ever reach the observer in the future and is about 5 Gpc.
The future horizon is when events become redshifted to arbitrarily long wavelengths in the exponentially expanding space.
I think the Hubble radius was the subject of the original question.
That’s why I seem to be getting fatter as I grow older … thank you … what a great excuse !!!
If everything is expanding, that would include all our devices that measure, and presumably all our measurement standards. So how is this expansion being quantified?
Just some rough arithmetic, but I come up with less than an attometer per second for two objects 1 meter apart … again very roughly an atomic nucleus diameter per hour … seems safely ignored …
I figured all that expanding is why it takes me longer to drive to work. I wish they’d update the highway mileage signs.
Read lazybratsche’s and Pantastic’s responses again. “Things” are not expanding, space is expanding. Things that held together by “strong enough” forces do not expand. The electromagnetic forces holding solid bodies together are strong enough by far to overcome the expansion of space. Even the relatively weak gravitational forces between stars in our galaxy is strong enough, so our galaxy is not expanding. The only thing that’s expanding is the space between galaxies that are distant enough to not have a strong enough gravitational attraction to each other.
–Mark
The Hubble surface doesn’t represent any kind of true horizon (i.e. limit on communication), except for when it coincides with another surface of interest.
In the standard cosmological model the Hubble surface is currently slowly moving away from us at a “speed” significantly less than c, in terms of proper distance and cosmological time. A necessary (but not sufficient) condition for the Hubble surface to be a true horizon is that it must be a null surface. If we assume a flat Universe, this leaves only two possibilities for its “speed”, either it must be:
a) zero, so that incoming light emitted at the Hubble surface remains at the Hubble surface
or
b) 2c, so that outgoing light emitted at the Hubble surface remains at the Hubble surface.
a) corresponds to a cosmological constant dominated Universe and b) corresponds to a radiation dominated Universe, and indeed in single-parameter models with those qualities the Hubble surface coincides with a horizon.
[quote=“Asympotically_fat, post:15, topic:761769”]
The Hubble surface doesn’t represent any kind of true horizon (i.e. limit on communication), except for when it coincides with another surface of interest.
That is true of any horizon, I disagree, while particles beyond the particle horizon could never of communicated in the past, and with the idea that there is no way to define a Hubble surface without a point of reference, those particles beyond that surface in opposite directions cannot communicate in the future. I see what you are getting at but outside of Newtonian style absolute space event horizons would be be non-sensical outside of a defined frame of reference.
Obviously in our time of the universes’ existence the Hubble surface is moving slowly but this will not always be true. (assuming we are in a forever expanding universe)
When we say there is a horizon we are relly saying that events within a certain spatial location lie outside either the past or future light cones of a certain (possibly conformal) event. For example the events bounded by black hole event horizon lie outside the past lightcone of future timelike infinity.
This has two consequences, firstly any horizon must be a null surface as all lightcones are null surfaces. Secondly for a given event there can only be at most two horizons* for a given event: one for the past lightcone and one for the future lightcone.
In cosmological spacetime, for a given event, there can be** two such cosmological horizons: the particle horizon, defined by the past lightcone of that event and the cosmological event horizon, defined by the future lightcone of that event.
In the standard cosmology, the Hubble surface coincides with neither of those horizons and is a timelike surface, which underlines the fact it is not a causal boundary. In particular we can communicate with particles lying in the (admittedly comparatively small) volume that lies outside the Hubble horizon, but inside the cosmological event horizon.
It is true that current events in one direction lying outside the Hubble surface cannot communicate in future with events in the other direction lying outside the Hubble surface. However in fact the range of communication is much less than this range of two Hubble radii, underlining that again the Hubble surface is not causally special.
In the standard cosmology the future evolution of the Hubble horizon is such that it is growing at a slow and increasingly slower speed such that it is “catching up” with the even-slower growing and slightly larger cosmological event horizon, such that the two are asymptotically approaching the same proper volume, which is only slightly larger than their two current volumes.
Of course all this is not to say that the Hubble surface is not a useful surface to talk about in certain contexts, but there is a lot of confusion about it actually being a causal boundary, which comes from the cases when it conincides or almost coincides with such a boundary.
*admittedly it is possible that such a horizon can be a disjoint surface, such as the event horizons of multiple black holes and if we were considering a set of events rather than a single event we might define more horizons.
**NB in certain models it is possible for the horizons to extend to infinity, in which case they don’t exist as such.
I have been told that we just become to short for our weight. However I cannot reconcile that with men who simultaneously become too tall for their hair.
One of life’s great mysteries, I guess.
Gravity, man… What’s the deal with that? Chronos?
Fucking gravity, how does it work?