James' World 2

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What will our Sun look like after it dies? Scientists have made predictions about what the end will look like for our Solar System, and when that will happen. And humans won’t be around to see the final act.

Previously, astronomers thought it would turn into a planetary nebula – a luminous bubble of gas and dust – until evidence suggested it would have to be a fair bit more massive.

An international team of astronomers flipped it again in 2018 and found that a planetary nebula is indeed the most likely solar corpse.

The Sun is about 4.6 billion years old – gauged on the age of other objects in the Solar System that formed around the same time. Based on observations of other stars, astronomers predict it will reach the end of its life in about another 10 billion years.

There are other things that will happen along the way, of course. In about 5 billion years, the Sun is due to turn into a red giant. The core of the star will shrink, but its outer layers will expand out to the orbit of Mars, engulfing our planet in the process. If it’s even still there.

One thing is certain: By that time, we won’t be around. In fact, humanity only has about 1 billion years left unless we find a way off this rock. That’s because the Sun is increasing in brightness by about 10 percent every billion years.

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(NASA/SDO)

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Steven Weinberg, who died on July 23, towered over theoretical physics in the second half of the 20th century. He strongly believed that, armed only with the fundamental principles of relativity and quantum mechanics, the theoretical physicist can examine all phenomena in the universe — from the smallest to the largest scales. His work transformed our understanding of every aspect of fundamental physics in startlingly deep and original ways.

Weinberg was a master of quantum field theory, a branch of physics born from applying the rules of quantum mechanics to the electromagnetic field, which sees a particle — the photon —as a “quantized” excitation of the field. He was instrumental in propelling quantum field theory to astonishing new heights in the description of nature.

The themes of unification and symmetry drove all of Weinberg’s work and led to his famous breakthrough on electroweak unification, which revealed a hidden unity between two of the universe’s four fundamental forces. At first sight, the electromagnetic and weak interactions seem utterly different: We see electromagnetic waves as light in everyday life, while the weak force — responsible for radioactivity — operates on subnuclear scales. Weinberg realized that at very high energies, the two forces should be intertwined, described by what’s known as Yang-Mills theory, whose equations have a special property called gauge symmetry. But this essential commonality is hidden by the so-called Higgs mechanism, which generates masses for elementary particles such as the electron and the W and Z particles (which mediate short-range weak interactions), all the while leaving the long-range photon massless. The model he proposed in 1967 for realizing this vision made many detailed predictions and has been triumphantly confirmed by experiments beginning in the 1970s and ’80s, capped off by the 2012 discovery of the Higgs particle. Weinberg shared the 1979 Nobel Prize in Physics with Sheldon Glashow and Abdus Salam for this work, a pillar of the Standard Model of particle physics.

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No matter where you look in the universe, in any direction, your line of sight will eventually run into some type of matter or radiation. The Earth is embedded in the solar system, with planets, moons, rocky and icy bodies, dust, and plasma particles permeating our environment. Beyond our own backyard are stars, gas, and dust strewn throughout the Milky Way, and at even greater cosmic distances are galaxies, quasars, and the matter in the intergalactic medium. If you somehow manage to pick a line of sight that doesn’t run into any of those, you’ll still encounter something: the cosmic microwave background, which is thought to be the leftover radiation from the early stages of the hot Big Bang.

And yet, no matter what we observe in any direction, two properties will correspond to whatever object we see:

1. We are seeing the object not as it is today, but as it was a finite amount of time ago: when it emitted the light that’s now striking our eyes.
2. That object is currently a specific distance away from us; if we could somehow “freeze” time and measure the distance between ourselves and that object, we’d get a certain value.

You might think that these two properties — time and distance — would be equal. A star whose light arrives after a journey of 10 years is 10 light-years away; a galaxy whose light arrives after a 100-million-year journey is 100 million light-years away; light from the Big Bang that arrives after a 13.8-billion-year journey has the emitting location 13.8 billion light-years away.

But that isn’t true at all — and the expanding universe is to blame.

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Artist’s logarithmic scale conception of the observable universe. Galaxies give way to large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. This ‘edge’ is a boundary only in time. (Credit: Pablo Carlos Budassi; Unmismoobjetivo/Wikimedia Commons)

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Where did all this come from? In every direction we care to observe, we find stars, galaxies, clouds of gas and dust, tenuous plasmas, and radiation spanning the gamut of wavelengths: from radio to infrared to visible light to gamma rays. No matter where or how we look at the universe, it’s full of matter and energy absolutely everywhere and at all times. And yet, it’s only natural to assume that it all came from somewhere. If you want to know the answer to the biggest question of all — the question of our cosmic origins — you have to pose the question to the universe itself, and listen to what it tells you.

Today, the universe as we see it is expanding, rarifying (getting less dense), and cooling. Although it’s tempting to simply extrapolate forward in time, when things will be even larger, less dense, and cooler, the laws of physics allow us to extrapolate backward just as easily. Long ago, the universe was smaller, denser, and hotter. How far back can we take this extrapolation? Mathematically, it’s tempting to go as far as possible: all the way back to infinitesimal sizes and infinite densities and temperatures, or what we know as a singularity. This idea, of a singular beginning to space, time, and the universe, was long known as the Big Bang.

But physically, when we looked closely enough, we found that the universe told a different story. Here’s how we know the Big Bang isn’t the beginning of the universe anymore.

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The modern cosmic picture of our universe’s history begins not with a singularity that we identify with the Big Bang, but rather with a period of cosmic inflation that stretches the universe to enormous scales, with uniform properties and spatial flatness. The end of inflation signifies the onset of the hot Big Bang. (Credit: Nicole Rager Fuller/National Science Foundation)

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