It's said that at the moment of the Big Bang, space expanded so that to an objective observer matter moved at thousands of times the speed of light. If so, how can we say the universe is 10-30 billion years old? Couldn't it be a billion years old with objects 10-30 billion light years away because of "riding the space wave"?

  • $\begingroup$ It doesn't work like that. The age of the universe is approximated by analyzing the cosmic background radiation. And the universe cannot be a billion years old as Earth itself is around 5 billion years old. $\endgroup$
    – Sam
    Commented Feb 1, 2020 at 4:27
  • $\begingroup$ When "universe" isn't capitalized, it usually refers to the "single universe" favored by many cosmologists, but often refers to one of the "local universes" in an inflationary multiverse, whose LU's are causally separated from each other. Purists about this matter will capitalize the word (as "Universe"), because the names of localities are capitalized in English: They include Poplawski, whose multiverse is past- and future-eternal, although its LU's aren't. (NASA recommends against such capitalization, but it's a transport agency that hasn't yet considered intergalactic travel.) $\endgroup$
    – Edouard
    Commented Oct 1, 2022 at 18:36

1 Answer 1


We estimate the current age of the universe by measuring the expansion of space and extrapolating backwards, taking into account the matter and energy density in the very early universe.

The universe is about 13.8 billion years old, and the observable universe has a diameter of roughly 93 billion lightyears.

The whole universe is likely infinite in size, and if so, it's always been infinite for all time $t>0$, where $t=0$ is the instant of the Big Bang; the size of the universe at $t=0$ is not mathematically well-defined. Please see Did the Big Bang happen at a point? for further details.

And even if the whole universe isn't infinite then current measurements of its global curvature indicate that it's most likely many times larger than the observable universe.

  • $\begingroup$ The question is only about paragraph 1. $\endgroup$
    – ProfRob
    Commented Feb 1, 2020 at 9:00
  • 1
    $\begingroup$ @RobJ Perhaps, although Eddie appears to be making some kind of correlation between the age of the universe and its size, so I felt it was relevant to discuss the size. $\endgroup$
    – PM 2Ring
    Commented Feb 1, 2020 at 9:15

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