When we measure how distant a variety of objects are from their physical and luminous properties — along with the amount that their light has been shifted by the Universe's expansion — we can come to understand what the Universe is made of.
Our cosmic cocktail, at present, consists of:. When you combine these effects together, you get a unique and unambiguous prediction for how far it is, at all times past and present, to the edge of the observable Universe.
This is Note the early radiation-dominated era, the recent matter-dominated era, and the current-and-future exponentially-expanding era. This is a big deal! Most people assume that if the Universe has been around for Only if the Universe were static and not expanding would this be true, but the fact is this: the farther away we look, the faster distant objects appear to speed away from us.
The rate of that expansion changes in a way that is predictable based on what's in the Universe, and in turn, knowing what's in the Universe and observing how fast objects expand tells us how far away they are.
When we take all of the available data together, we arrive at a unique value for everything together , including the distance to the observable cosmic horizon: The observable Universe might be 46 billion light years in all directions from our point of view, Over time, we'll be able to see more of it, eventually revealing approximately 2. This boundary, however, is not an "edge" to the Universe in any conventional sense of the word. It is not a boundary in space at all ; if we happened to be located at any other point in space, we would still be able to detect and observe everything around us within that This is because that "edge" is a boundary in time, rather than in space.
This edge represents the limit of what we can see because the speed of light — even in an expanding Universe governed by General Relativity — only allows signals to travel so far over the Universe's This distance is farther than However, we cannot reach all of it.
The size of our visible Universe yellow , along with the amount we can reach magenta. If we Beyond a certain distance, we can see some of the light that was already emitted long ago, but will never see the light that is being emitted right now: Beyond a certain specific distance — calculated by me to be approximately 18 billion light-years away at present — even a signal moving at the speed of light will never reach us.
Similarly, that means that if we were in an arbitrarily high-powered rocket ship, all of the objects presently contained within this 18 billion light-year radius would be eventually reachable by us, even as the Universe continued to expand and these distances continued to increase.
However, the objects beyond that would never be reachable. At this point, they can be million miles million km apart. Related: What is the Temperature on Mars? Light travels at approximately , miles per second , km per second. Therefore, a light shining from the surface of Mars would take the following amount of time to reach Earth or vice versa :. In January , the probe left Earth at 36, mph 58, kph.
If such a probe traveled in a straight line to Mars, the time it would take to get to Mars would be:. Of course, the problem with the previous calculations is that they measure distance between the two planets as a straight line. Traveling through the farthest passing of Earth and Mars would involve a trip directly through the sun, while spacecraft must of necessity move in orbit around the solar system's star. Although this isn't a problem for the closest approach, when the planets are on the same side of the sun, another problem exists.
The numbers also assume that the two planets remain at a constant distance; that is, when a probe is launched from Earth while the two planets are at the closest approach, Mars would remain the same distance away over the course of the 39 days it took the probe to travel. In reality, however, the planets are continuously moving in their orbits around the sun. Engineers must calculate the ideal orbits for sending a spacecraft from Earth to Mars.
Their numbers factor in not only distance but also fuel efficiency. The whole point is to break the transmission path, she says, and limiting social contact and continuing protective behaviours such as masking can help to reduce the spread of new variants while vaccines are rolling out. Texas and some other US state governments are already lifting mask mandates, even though substantial proportions of their populations remain unprotected. Even after the threshold has been passed, isolated outbreaks will still occur.
To understand the additive effects of behaviour and immunity, consider that this flu season has been unusually mild. This back-of-the-envelope calculation shows how behaviour can change the equation, and why more people would need to be immunized to attain herd immunity as people stop practising behaviours such as social distancing.
Ending transmission of the virus is one way to return to normal. Even without herd immunity, the ability to vaccinate vulnerable people seems to be reducing hospitalizations and deaths from COVID The disease might not disappear any time soon, but its prominence is likely to wane. News 11 NOV Career Guide 10 NOV Article 10 NOV News 04 NOV News Explainer 02 NOV Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily. Advanced search.
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