~ the large Bang, the world was practically perfectly uniform, and also full the matter, power and... <+> radiation in a rapidly widening state. As time go on, the cosmos not only creates elements, atoms, and molecules which clump and also cluster together, leading to stars and also galaxies, yet expands and also cools the whole time.

You are watching: How old is the universe in light years


If the universe is 13.8 billion years old, and the speed of light is important our cosmic rate limit, how far away have to we be able to see? The answer appears obvious: 13.8 billion light-years, because a light-year is the distance light deserve to travel in a year, and nothing deserve to go quicker than that.

Unfortunately, like a an excellent many answers the seem evident when you use your logicalcommon feeling to them, that"s not just how things in reality work. In reality, if you were to look in ~ the most remote thing of all you deserve to possibly see, and ask "how far away is it," the price is much farther than that: 46 billion light-years. That might sound impossible, but it"s not. Friend just have actually to increase your method of thinking.

The initial conception that space, many thanks to Newton, together fixed, absolute and unchanging. It to be a... <+> stage where masses could exist and attract.

Amber Stuver, from her blog, life Ligo

Traditionally, the means you most frequently think that a street is by taking 2 points and drawing a line between them. It"s other we discover to carry out as kids, and keep v us into adulthood. For many applications, there"s no difficulty in doing this, whether we use a ruler, one odometer, or a light clock: by measuring the lot of time the takes a light signal to take either a one-way or round-trip journey.

But this presumption isn"t strictly valid once it pertains to the Universe. Distance isn"t necessarily characterized by a straight line, nor execute those distances remain the same over time. The reason for this is something us don"t think about in our day-to-day experience: space isn"t flat, and it"s likewise inextricably attached to time, in the form of spacetime.

The gravitational habits of the Earth approximately the sun is not as result of an invisible gravitational... <+> pull, however is far better described through the earth falling openly through curved an are dominated by the Sun. The shortest distance in between two clues isn"t a straight line, yet rather a geodesic: a bent line that"s identified by the gravitational deformation the spacetime.

LIGO/T. Pyle

The "space isn"t flat" part is perhaps less complicated to understand. When you think about the planet revolving about the Sun, you probably think about it the same method that Newton did: in terms of an invisible, attractive pressure acting from one thing (the Sun) on one more (the Earth).

This is the way we thought about gravity for centuries, and also it literally take it a genius in ~ the level of Einstein to go past it. That isn"t that mass at a particular distance causes a force, but that mass is a form of energy, and also energy causes the cloth of the world to curve. The cloth of the universe isn"t just space, however a quantity recognized as spacetime, whereby anyone and also anything in the experiences room and time together, dependence on how they"re moving relative to whatever else in the Universe.

In a cosmos that isn"t expanding, you can fill that with issue in any type of configuration you like, but... <+> the will constantly collapse down to a black color hole. Such a world is unstable in the context of Einstein"s gravity, and must be expanding to be stable, or we should accept its inevitable fate.

E. Siegel / past The Galaxy

One of the things we learn around a cosmos governed by Einstein"s laws— basic Relativity— is the it can not be both static and stable if it has actually matter in it. A cosmos that"s static, where the overall fabric of spacetime doesn"t readjust over time, would be in problem if you put matter down into it. Over time, that matter would gravitationally attract, and would draw itself with each other towards a point. In a static universe filled through matter, there"s just one feasible fate: contracting down to a black hole.

Don"t worry; that"s no our fate.

The "raisin bread" model of the broadening Universe, where relative ranges increase as the space... <+> (dough) expands. The aside from that away any kind of two raisin room from one another, the greater the it was observed redshift will be by time the irradiate is received.

NASA / WMAP scientific research Team

Because our world is act the one thing it have the right to do to avoid it: it"s expanding. The best means to imagine the cosmos is together a bread of dough in some zero-gravity oven, wherein the dough is filled v raisins.

Each separation, personal, instance raisin to represent a gravitationally bound framework in the Universe: a star cluster, a galaxy, a group of galaxies, or something even larger. Each raisin also isn"t bound to any type of other raisin; they room far sufficient apart the gravity will certainly not carry them together, also given an infinite amount the time.

Why? because the dough is rising. And that dough to represent the towel of spacetime. As time goes by, the universe expands, and also distant raisins (galaxies) appear to move away native one another.

The balloon/coin analogy that the broadening Universe. The individual structures (coins) don"t expand,... <+> yet the distances between them execute in an expanding Universe. This deserve to be an extremely confusing if you firmly insist on attributing the evident motion of the objects we see to their loved one velocities through space. In reality, it"s the space between castle that"s expanding.

E. Siegel / beyond The Galaxy

This is the vital point that"s so hard for most civilization to understand. The growth of the cosmos isn"t around a speed. The universe doesn"t expand at the speed of light, the speed of sound, or any other speed. If you were to look in ~ a raisin that"s close by you, that would show up to move away native you relatively slowly, and also a light signal sent out from it come you would only take a brief amount that time to get there. Yet if you were to look at a raisin that was much farther away, it would show up to recede much more quickly. A irradiate signal sent from it come you would certainly take a very long time to obtain there.

The reason is because the development of the Universe counts on how much away an object is native you. It"s no a speed; it"s a speed-per-unit-distance.

Radiation gets redshifted together the universe expands, definition it was more energetic in the Universe"s... <+> past, with a higher amount of energy per photon. Even if it is the world is overcame by issue or radiation is irrelevant; the redshifting is real.

E. Siegel / past The Galaxy

This is why, as soon as we talk about the measured development rate that the Universe— what us sometimes speak to the Hubble constant— the comes together with such weird, international values: something favor ~70 km/s/Mpc. This tells united state that because that every megaparsec (Mpc, or about 3.26 million light-years) a galaxy is remote from any kind of other galaxy, it shows up to recede in ~ 70 km/s.

So if an object is presently 100 Mpc far from us, it appears to move away in ~ 7,000 km/s.

If an item is 4,300 Mpc far from us, it shows up to relocate away at approximately 300,000 km/s, or the rate of light.

And if things is 14,100 Mpc away from us, it appears to move away at around 987,000 km/s, i beg your pardon is a crazy big number.

The distance/redshift relation, including the most distant objects that all, checked out from their form Ia... <+> supernovae. The data strong favors an accelerating Universe. Note exactly how these lines are all various from one another, together they correspond to Universes make of different ingredients.

Ned Wright, based upon the latest data native Betoule et al.

But I save saying miscellaneous you might be glossing over: itappears the these objects move away from united state at this speeds. In reality, the objects us aren"t moving, just like the raisins aren"t moving relative come the dough that they"re in. Instead, what"s continue is that the fabric of spacetime chin is expanding, and the irradiate coming from these objects is acquiring stretched— come longer, red wavelengths— as the cosmos expands.

This is why us talk around the redshift of remote objects: due to the fact that their light gets extended as the towel of the world expands. It"s the matter and energy density of the cosmos that determines how conveniently the cosmos expands, and also we have actually to add up every the different types of energy, consisting of neutrinos, radiation, dark matter and also dark energy, to get the right answer.

it isn"t simply that galaxies are relocating away from us that causes a redshift, yet rather that the... <+> space between ourselves and the galaxy redshifts the irradiate on its trip from the distant point to our eyes. This influence all develops of radiation, including the leftover glow from the big Bang.

Larry McNish / RASC Calgary facility

Today, there"s light getting here at our eyes from every sorts of different objects at all sorts of different distances. The objects that are 13.8 billion light-years far from us currently were lot closer in the remote past. As soon as they very first emitted the irradiate that"s reaching us today, this occurred at a time the was currently billions of years ago. That galaxy can be 13.8 exchange rate light-years away ideal now, yet the light didn"t should travel because that 13.8 billion years to with us; it traveled a shorter distance and also for a much shorter amount that time.

In fact, we have the right to see objects that are farther away 보다 13.8 billion light-years today, all since of the fact that the fabric of the world itself is expanding.

So what perform we carry out if we desire to recognize how big the observable cosmos is? We should ask the following question:

Given all we know around the broadening Universe and what the different amounts of all the different species of power that space in it are, how much away would things be today if the light were only, simply now, arriving after a journey of 13.8 exchange rate years?

If you execute the math, you get an incredible answer: 46 exchange rate light-years. (Or 46.1 billion light-years if you desire to be even an ext precise.) If our universe had an ext dark energy and less matter, the answer would be slightly larger; if the world had an ext matter and also less dark energy, the answer would be contempt smaller. However that"s how we gain to the leaf of the observable Universe.

within the observable universe (yellow circle), there are roughly 2 sunshine galaxies.... <+> Galaxies an ext than about a 3rd of the means to the border of what we deserve to observe deserve to never it is in reached as result of the Universe"s expansion, leaving only 3% the the Universe"s volume open up to human exploration. However, we can still check out the galaxies beyond that, other than we"re minimal to see them as they to be in the past.

Wikimedia Commons individuals Azcolvin 429 and Frédéric MICHEL / E. Siegel

This doesn"t average we deserve to reach everything in the part of the cosmos we can see! The most remote parts of the cosmos are only visible throughout the more quickly stages. In fact, whatever that"s more distant than around 4,300 Mpc (or 14 exchange rate light-years) this particular day is in ~ the border of how far we deserve to reach at the rate of light. The object much more distant than that can still be seen by us, but only as they were in the past; similarly, they have the right to only see us as us were in ours past. Someone more distant 보다 14 billion light-years indigenous us, also with one infinitely an effective telescope, can never watch human world as that is this particular day on Earth.

A graph the the size/scale that the observable world vs. The passage of cosmic time. This is... <+> displayed on a log-log scale, through a couple of major size/time milestones identified. Note the early radiation-dominated era, the recent matter-dominated era, and the current-and-future exponentially-expanding era.

E. Siegel

The truth that we deserve to see the world we do tells us that it have to be expanding, a fantastic match of theory and observation. It likewise tells united state that we deserve to extrapolate back in time come as at an early stage a phase as we want, and find all sorts of exciting milestones that take place as far as the size of the universe is involved compared v its age.When the cosmos was a million year old, its edge was already some 100 million light-years away. When it was just a year old, we could see for virtually 100,000 light-years. When it was simply a millisecond old, us could currently see because that a light-year in all directions.

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And today, 13.8 billion years after the large Bang, the farthest thing we might possibly see, corresponding to the light emitted at the first moment the the large Bang, is 46.1 exchange rate light-years distant. Given the components of our Universe, that couldn"t have turned out any other way.


I am a Ph.D. Astrophysicist, author, and science communicator, who professes physics and also astronomy at assorted colleges. I have actually won numerous awards for scientific research writing

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I am a Ph.D. Astrophysicist, author, and also science communicator, that professes physics and astronomy at various colleges. I have actually won numerous awards for science writing because 2008 for my blog, Starts v A Bang, consisting of the award for ideal science blog by the institute of Physics. My two books, Treknology: The scientific research of Star Trek indigenous Tricorders to Warp Drive, beyond the Galaxy: how humanity looked beyond our Milky way and found the entire Universe, are accessible for purchase at Amazon. Follow me top top Twitter