### Does the mass of the observable universe ever change?

• First do we have anyway to even estimate the mass of the entire observable universe? And then is there any data that shows mass being gained or lost? Would we ever know if someone was playing with the til.

Also I want to be clear that I am not talking about small masses on the outskirts of the "universe" or small discrepancies in measurement or anything of the sort.

Note: I would like to add that maybe we should define the observable universe as NOW (x-date) so that we aren't calculating a moving target.

Mass is due to higgs boson and I think you are referring to matter and energy of any kinds.

conservation of mass and energy: http://www.lightandmatter.com/html_books/7cp/ch01/ch01.html It's not known whether the universe is a closed system.

Are we able to measure the mass of the whole universe? That puts a wrinkle into the question. My guess would be no because there's no known method and no observation for the creation of new mass that I know of, unless dark energy has mass, in which case, the answer would probobly be yes.

@user6760 That's not quite true. Only some mass comes from the Higgs mechanism, namely the mass of the $W^{\pm}$ and $Z$ bosons for the weak force. The proton, however, only gets 1% of its mass from its constituent quarks, and the quark masses are intrinsic, rather than derived from the Higg's mechanism. The rest comes from the quarks' kinetic energy and the strong force binding them.

Somewhere out there is the very edge of the observable universe, and at that edge, there'll be hydrogen atoms completely oblivious to the fact that they're receding from us at near lightspeed. They're just drifting around with the usual assortment of local, random velocities. When two such atoms collide, one might be kicked up to above lightspeed relative to us and disappear from the observable universe. That'd lower the observable mass. It's just a little kinetic thing across a barrier, but we might even pick up a photon from collisional ionization in such an event.

@WayfaringStranger: Most of the Universe recedes from us faster than light, and in fact the "edge" of the observable Universe recedes at more than $3c$.

Your latest update to the question confuses me: If you define the observable Universe as _now_, then obviously its mass doesn't change, since you have just frozen the Universe in time. If your intention is to disregard the mass increase due to matter entering the horizon, then the total mass still increases because the volume, and hence the amount of dark energy, increases. Normal matter and dark matter is neither created nor destoyed, but light and other relativistic particles are redshifted and lose energy / mass.

I agree, this a complete shift of the goal posts. Your question is now unclear.

@pela - I am not freezing the universe I am freezing the area of reference. If you think there is a better way to have an apples for apples comparison I can reword. I don't want the question to be a reflection of what is entering or exiting our observation ability. Maybe this is impossible since we are all moving targets but it would be nice to remove as many variables as we can.

Okay, but if you freeze the area of reference then either 1) you fix the volume in physical coordinates and galaxies will leave your volume at $\sim3\times$ the speed of light, or 2) you fix the volume in comoving coordinates and dark energy is created (and light is redshifted). Does this make sense? I think you need to decide what it is you actually want to know. Do you want to know whether mass/energy is continuously created/destroyed? The total mass of the Universe right now is known, and discussed in my answer below.

• pela Correct answer

7 years ago

Even if you're only referring the "ordinary" matter (such as stars, gas, and bicycles) and dark matter, the mass of the observable Universe does increase, not because mass is being created, but because the size of the observable Universe increases. In a billion years from now, we can see stuff that today is too far away for the light to have reached us, so its radius has increased. Since the mass $M$ equals density $\rho_\mathrm{M}$ times volume $V$, $M$ increases.

As called2voyage mentions, we have several ways of measuring the density, and we know it's close to $3\times10^{-30}\,\mathrm{g}\,\mathrm{cm}^{-3}$. The radius is $R = 4.6\times10^{28}\,\mathrm{cm}$, so the mass is
$$M = \rho_\mathrm{M} \frac{4\pi}{3}R^3 \simeq 10^{57}\,\mathrm{g},$$
or $5\times10^{23}M_\odot$ (Solar masses).

However, another factor contributes to the mass increase, namely the so-called dark energy, which is a form of energy attributed to empty space. Since the Universe expands, dark energy is being created all the time.

You used the radius of about 46 billion light years, the current estimate and the answer below, Dr. Jagadheep Pandian used 13.8 billion light years so you came up with answers that were off by about a factor of 30 using the same density. I'm curious which one is correct. My guess is that yours is, but I'm curious to have that verified.

@userLTK: Yes, it seems Jagadheep has an error in his description. Although the Universe is 13.8 billion years (Gyr) old, we can see farther than 13.8 billion lightyears (Gly) away, because the Universe has expanded in the meantime. The exact result can be found by integrating (numerically) the Friedmann equation over time, and turns out to be roughly 46.5 Gly. Since incidentally, this result can also be expressed as roughly 14 billion parsec (Gpc), it might also be this that confused Jagadheep.