Space · Foundations
The universe is about 13.8 billion years old—and the region observable today is roughly 93 billion light-years across. This apparent contradiction takes us straight into modern cosmology: light travels through space that changes along the way.
Our Cosmic Horizon Is Not an Outer Wall
To understand the size of the universe, we must first distinguish between the observable universe and the universe as a whole. The observable universe is the region from which information has been able to reach us since early cosmic history. Its present-day radius is approximately 46 billion light-years, and its diameter is around 92 to 93 billion light-years. The small difference between commonly quoted figures is not a fundamental contradiction at this level of rounding. This boundary is not a photographed wall or a confirmed edge of space. It describes an observational horizon. We know much less about the total size of the universe. It could be far larger than the portion we can see; whether it is spatially finite or infinite remains an open question. Even a telescope with higher resolution cannot conjure up information that has not yet reached us. Saying “the universe is 93 billion light-years across” would therefore be too sweeping: that figure applies to the observable region.
Why Size and Age Are Consistent
According to the standard cosmological model, the universe is approximately 13.8 billion years old. This age is inferred from observations and a model of its evolution, particularly from the cosmic microwave background and cosmic expansion. It is not a date that could be read from a single stellar clock. It might seem natural to expect a radius of no more than 13.8 billion light-years. That would, however, assume that space does not change. While ancient light was traveling toward us, cosmic distances increased. The regions from which that light originated are therefore much farther away today than a simple multiplication of the speed of light by travel time would suggest. Present-day distance, distance at the time of emission, and light-travel time are different quantities. Across cosmic distances, they cannot be used interchangeably.
Expansion per Minute: A Number Needs a Distance
There is no single speed for the entire universe that answers the question of how much it grows per minute. Today's expansion is described by the Hubble constant. In simplified terms, the contribution from cosmic expansion follows this relation: speed = Hubble constant × present-day distance. The greater the distance between regions moving with the cosmic expansion, the greater the current increase in that distance. For an illustrative calculation, we will use a deliberately rounded value of 70 kilometers per second per megaparsec. One megaparsec is approximately 3.26 million light-years. At a separation of one megaparsec, this gives an increase of approximately 4,200 kilometers per minute. At ten megaparsecs, the increase is 42,000 kilometers per minute; at a hundred megaparsecs, it is 420,000. These are our own conversions of the chosen rate, not three independently measured values. The unit matters: 4,200 kilometers per minute per megaparsec is not an acceleration of “4,200 kilometers per minute every minute.” Nor does every object expand along with the universe. Atoms, people, Earth, and gravitationally bound systems such as the solar system do not simply follow cosmic expansion. In our neighborhood, peculiar motions and gravity can be superimposed on the contribution from expansion.
Sources: [2]
The Hubble Tension and Accelerating Expansion
The current expansion rate has not been pinned down to one undisputed value. Inferences from the early universe and the standard model yield approximately 67 to 68 kilometers per second per megaparsec; local distance measurements often give values around 73. This discrepancy is called the Hubble tension. The value of 70 used for the calculation in the previous section is a teaching choice, not an average that resolves the research problem. This must be distinguished from the observation that cosmic expansion has been accelerating during the universe's more recent history. In the standard model, this is described by dark energy. It does not automatically mean that the Hubble constant itself increases over time: it is a relative expansion rate, whereas “accelerating” refers to the evolution of the cosmic scale factor. Such distinctions illustrate why intuitive language sometimes needs more precise definitions. At very large separations, the rate at which cosmic distances change can also exceed the speed of light. This is not the local motion of a spacecraft through space at faster-than-light speed. Special relativity limits local signal speeds; changes in cosmic distances within an expanding spacetime require different treatment. The universe's unknown future cannot be determined from this with a single linear extrapolation.
Sources & further reading
- [1] NASA: How Big Is Space?nasa.gov
- [2] NASA/Hubble: The Hubble Constant and the Hubble Tensionscience.nasa.gov
- [3] NASA: Building Blocks of the Universescience.nasa.gov
Written in our own words with AI assistance, based on the sources listed above. Numbers are rounded; estimates, model assumptions, and open research questions are explicitly put into context. A foundational article, not a daily news report.