Did light exist at the beginning of the universe?
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Nowadays, the dark of night is interspersed with the light of stars. But before the stars were born, did light shine at the beginning of the universe?
The short answer is "no." But the long answer reveals light's extraordinary journey. At first, the early universe's light was "trapped," and it took several hundred thousand years for it to escape. Then, it took about 100 million years for stars to form.
By examining the speed and direction in which galaxies were moving, astronomer Edwin Hubble discovered the universe was expanding. This 1929 discovery suggested that the cosmos was once smaller, with scientists eventually calculating that the entire universe was concentrated into one, infinitely dense point about 13.8 billion years ago, until the Big Bang happened.
"With the Big Bang, space was created and expanded, along with everything in the universe," Andrew Layden, chair of physics and astronomy at Bowling Green State University in Ohio, told Live Science.
The only way all the matter that now makes up the universe could fit in a tiny spot "is if it was energy at that time," Layden said. Einstein's famous equation E=mc2 revealed that energy and mass can be interchangeable, Layden explained.
As the universe expanded, the density of its energy decreased, and it cooled. The first particles then began to form within the first second after the Big Bang, according to Las Cumbres Observatory. These included the photons that make up light, as well as the protons, neutrons and electrons that make up atoms. By about three minutes after the Big Bang, protons and neutrons could fuse together to create the nuclei of atoms such as helium, according to NASA.
"Think of fog and dew," Layden said. "Particles in a high-energy state are dispersed like water in fog, and when the energy gets low enough, they can condense out like droplets of dew."
Related: Can anything travel faster than the speed of light?
However, although photons of light existed since the first second after the Big Bang, they could not yet shine across the universe. This is because the early cosmos was so hot that "electrons were moving too fast for atomic nuclei to hold them in orbit around them," Layden said. "The universe was just this very hot, dense soup."
All the electrons zipping around freely in the early universe meant that light could not move around very much. "As light tried to travel in a straight line during this time, it always bumped into electrons, so it could not go very far," Layden said.
A similar situation is found within the sun, Srinivasan Raghunathan, a cosmologist at the University of Illinois, Urbana-Champaign, told Live Science. "You can imagine a photon of light created by nuclear reactions at the center of the sun trying to come out to the sun's surface," he said. "The center of the sun is extremely hot, and so there are a lot of free electrons present. This means light cannot travel in straight lines."
The distance from the center of the sun to its surface is about 432,450 miles (696,000 kilometers). The speed of light in a vacuum is about 186,000 miles per second (300,000 km/s), but in the sun, "it takes about 1 million to 2 million years for light to escape from the center of the sun to its surface," Raghunathan said.
However, about 380,000 years after the Big Bang, the expansion of the universe let the cosmos cool enough for atomic nuclei to glom onto electrons. "When that happens, all those electrons are no longer free," Layden said. "This happens at about 3,000 Kelvin [4,940 degrees Fahrenheit, or 2,725 degrees Celsius], the surface temperature of a coolish reddish star."
Within a short number of years, "everything goes from being a hot dense soup to a clear universe where light can travel freely," Layden said. "At that moment, the first photons in the universe can escape."
The light typical of the universe when it was about 3,000 kelvins was in near-infrared to visible wavelengths, Layden noted. However, as the cosmos expanded over the course of more than 13 billion years and cooled to an average temperature of about 2.73 Kelvin (minus 455 F, or minus 270 C), the universe's first light stretched to longer microwave wavelengths.
Astronomers first detected this leftover radiation from the Big Bang, called the cosmic microwave background, in 1964.
RELATED MYSTERIES
—What is the smallest particle in the universe? (What about the largest?)
—What would happen if the speed of light were much lower?
—Where do electrons get energy to spin around an atom's nucleus?
Analyzing these microwaves has yielded many insights. For instance, the gravitational pull of galaxies can distort light — a phenomenon called gravitational lensing. Examining the amount of distortion the cosmic microwave background has experienced at different points in the sky can help scientists reconstruct the large-scale structure of the universe — the arrangement of galaxies and the giant voids between them across the cosmos, Raghunathan said.
After the light from the Big Bang was released, the universe experienced a period known as the cosmic dark ages. Eventually, after millions of years, the gravitational pull of clouds of gas led these clumps of matter to collapse in on themselves.
"This created the first generation of stars, and the universe had galaxies full of stars by about 1 billion years after the Big Bang, beginning the cosmic dawn," Layden said.

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Left Image: CGM in the self-injector. Right image: CGM on the back of my right arm. Nasha Addarich Martínez/CNET From the Oura app, I could see how my glucose levels fluctuated throughout the day. You can easily access it at the top of the app, where you also see other data metrics like sleep, heart rate, activity and readiness score (a personalized score, ranging from 0 to 100, that provides a snapshot of how well your body has recovered from the previous day's activities and sleep). How even my 'healthy' food choices raised my blood sugar (and that's OK) One of the most eye-opening parts of my testing was seeing how some of my favorite healthy staples played out on the glucose graph. A great example of this was my favorite pressed juice. I had a pressed juice in the morning that contained pineapple, beets, carrots, ginger, cucumber and lemon. It's nutrient-dense, but it also spiked my glucose above what's considered my healthy range for about 6 minutes (a small window and still considered optimal for my age and overall metabolic health). This was enough to make me rethink how and when I consume fruit-heavy beverages. The next day, I experimented with having a fruit smoothie instead of juice. I made a homemade smoothie with berries, banana, plant-based protein powder, peanut butter, soy milk and oats. This smoothie kept my glucose completely in range (zero minutes above my personal threshold). The added protein and fiber from the fruit, oatmeal, peanut butter and protein powder helped slow the absorption of natural sugars into my bloodstream. Nasha Addarich Martínez/CNET What I learned: Even natural fruit juices can cause glucose spikes, especially when consumed on an empty stomach and without any protein or fiber. Smoothies that include protein, fat and fiber can help keep spikes at bay and help stabilize your energy levels. Balance your meals with protein, fats and fiber Nasha Addarich Martínez/CNET Later that week, I had sushi rolls with a side of edamame for dinner. To my surprise, my glucose levels stayed within range the whole time. Oura Advisor (Oura's AI coach) even pointed out that this food combination offered protein and fiber, which likely contributed to the steady glucose levels. This moment was really encouraging for me. It made me realize that I don't need to drastically change what I eat. By just being more intentional about how I pair my foods, I can drastically improve the impact they have on my blood glucose. Pairing your foods with healthy fats like avocados, nuts, seeds and olive oil can also slow digestion and therefore the absorption of sugars. 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One of the more surprising things I learned was that blood sugar stability isn't just about what you eat. It's also about what you do after you eat. One afternoon, I had a slice of cake for a snack. No fiber or protein added, just the cake (I had lunch a few hours before). Instead of my blood sugar spiking, my glucose levels remained steadily in range. The caveat? I took a brisk walk after having my dessert. It wasn't anything strenuous, just a short 15-minute walk around my neighborhood. But it was enough to help my body process and use the glucose more efficiently. "The body knows that exercise demands more energy, and so it helps open up our cells to soak up sugar (energy) and remove it from our blood. A 10-minute walk burns about 30 to 40 calories, and your body will get that energy from circulating glucose or stored glycogen," Fitton explained. After eating cake and going for a walk, my blood sugar saw a slight spike, but stayed within range. Nasha Addarich Martínez/CNET Taking a walk after meals has long been recommended by experts for metabolic health, but seeing the effect in real time made it click for me. I do think that having the cake after having a balanced meal with protein and fiber also contributed to my steady levels of blood sugar. Sometimes, it's not about completely eliminating our favorite treats or carbs, but more so about timing, food pairing and movement. 3 ways I learned to flatten my glucose curve without sacrificing my favorite meals Before using the glucose sensor with Oura, I assumed blood sugar spikes were mostly about foods and how they impact your body. I also thought the only way to manage them was to cut carbs or drastically change my diet. But over these two weeks, I experienced firsthand how and when I eat (and move) matter just as much as what I eat. I learned that these three strategies helped keep my glucose levels in check. Pair carbs and sweets with protein, fiber and healthy fats When I had a berry smoothie with protein powder and soy milk, my glucose levels stayed steady. In contrast, when I had a fruit smoothie earlier that week, my blood sugar levels spiked and crashed more quickly. Takeaway: Carbs on their own, especially refined and processed carbs, can spike blood sugar quickly, but adding protein or fat slows digestion and leads to a more gentle spike. Food pairing examples: Apples and peanut butter, toast with avocado, rice with beans or another it out (especially after sweets) When I had cake as a snack, I was sure my graph would spike dramatically, but it didn't because of my walk (and eating a balanced meal before). A simple walk, even just 10 to 15 minutes, can help offset blood sugar spikes. This small but simple habit helps me now enjoy treats without the crash later. Timing your food matters I also noticed that when I ate higher-carb foods earlier in the day, my body processed them better than when I had them later at night. Breakfast smoothies and bowls kept my levels stable, while the same smoothie after 9 p.m. caused more spikes. "When it comes to blood sugar management, we see that in the morning our body usually has a spike in cortisol (a stress hormone) that can actually help our body take up sugar very quickly and quickly manage blood sugar. Eating in the morning will also calm the cortisol response, reducing the stress on your body. If you wait too long to eat, excess cortisol can create inflammation and also produce larger glucose spikes when you do eat," explained Fitton. Fitton further explained that, conversely, in the evening, the sugar management systems in our bodies start powering down and are not as good at helping blood sugar get into the cells. So it is important to try not to eat too late; otherwise, you'll probably see blood sugar go up and stay up for longer. That doesn't mean you can't eat carbs or treats at night, but eating them earlier in the day when insulin sensitivity tends to be higher can make a real difference.