How Fast is the Universe Expanding?

Article Updated: 23 Dec , 2015

The Universe is expanding, but how quickly is it expanding? How far away is everything getting from everything else? And how do we know any of this anyway?

When astronomers talk about the expansion of the Universe, they usually express it in terms of the Hubble parameter. First introduced by Edwin Hubble when he demonstrated that more distant galaxies are moving away from us faster than closer ones.The best measurements for this parameter gives a value of about 68 km/s per megaparsec.

Let’s recap. Hubble. Universe. Galaxies. Leaving. Further means faster. And then I said something that sounded like “blah blah Lando blah blah Kessel Run 68 km/s per megaparsec”. Which translates to if you have a galaxy 1 megaparsec away, that’s 3.3 million light years for those of you who haven’t seen Star Wars, it would be expanding away from us at a speed of 68 km/s. So, 1 megaparsec in distance means it’s racing away at 68 km/s.

This is all because space is expanding everywhere in all places, and as a result distant galaxies appear to be expanding away from us faster than closer ones. There’s just more “space” to expand between us and them in the first place. Even better, our Universe was much more dense in the past, as a result the Hubble parameter hasn’t always had the same value.

There are two things affecting the Hubble parameter: dark energy, working to drive the Universe outwards, and matter, dark and regular flavor trying to hold it together. Pro tip: The matter side of this fight is currently losing.

Expansion of the Universe. Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.

Expansion of the Universe. Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.

Earlier in the Universe, when the Hubble parameter was smaller, matter had a stronger influence due to its higher overall density. Today dark energy is dominant, thus the Hubble parameter is larger, and this is why we talk about the Universe not only expanding but accelerating.

Our cosmos expands at about the rate at which space is expanding, and the speed at which objects expand away from us depends upon their distance. If you go far enough out, there is a distance at which objects are speeding away from us faster than the speed of light. As a result, it’s suspected that receding galaxies will cross a type of cosmological event horizon, where any evidence of their existence, not even light, would ever be able to reach us, no matter how far into the future you went.

What do you think? Is there anything out there past that cosmological event horizon line waiting to surprise us?

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5 Responses

  1. dimar says:

    I wonder how would we perceive the universe if we/humans were smaller than an atom? Would everything be totally different?

  2. Tom2moro says:

    I appreciate your explanation, which confirms what I have read before about space expanding, but you did not explain what I have not been able to learn as yet: Why is space expansion occurring only far out but not locally?
    Is it not noticeable at our local rate, or what?

    • john kulick says:

      According to the “mainstream” description of the expansion, the expansion of spacetime stops at the boundary of gravitationally bound galaxies. Atoms are even more tightly bound so they would be even more resistant to expanding.
      A common example you would see in an astronomy book describing the expansion is that of buttons glued on to an expanding balloon; the buttons represent the fixed sized galaxies and the expanding surface represents a dimensionally reduced example of the expansion of spacetime.
      If the expansion of spacetime included matter, or was a continuous feature assocated with space, even the space within the atom, it would take about 14 x 10^9 years for a meter stick to double in size. This, for the most part, would be very difficult to measure.
      (I have developed a fairly convincing model which allows the expansion of spacetime to include the space within the atom, but that is not what you were asking about, but it is the reason I am familiar with the “standard ” model.)

    • Sean Nelson says:

      ” Why is space expansion occurring only far out but not locally?”
      Actually the author here didn’t say that he said ” space is expanding everywhere in all places”

  3. Peter Wilson says:

    How fast is the universe expanding?
    Picture 100 Mly of space the size of a beach-ball. Wait a million years. Expanding at the Hubble rate of 68 km/s per megaparsec, the beach-ball will have expanded by 1/10th of a mm, about the thickness of plastic wrap.

    For comparison, the Moon’s orbit is expanding at the rate of 100 km/s per megaparsec. That sounds fast, but we wouldn’t know that without the most precise measurement’s of its distance imaginable. The expansion of the cosmos is glacial. As with the cosmos, the expansion of the Moon’s orbit is accelerating.

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