Are we alone in the universe?

 Doubt it, there are 100 billion stars in our galaxy, and about every 5 of them have planets orbiting them. And there are many galaxies, numbers become meaningless, and there are many that are thousands of times larger than the Milky Way. But it is big, the distances are enormous, but I think it is unlikely that only a tiny fraction of one of them has intelligent life on it, although I question whether humans are as intelligent as we think, then we had not polluted our thin atmosphere with carbon that warms the climate to a limit that will soon be irreversible, so we wipe out the basis of life on the planet, for what, that a few people should have enormous incomes. Meaningless. So I sincerely hope that we are not alone, because then the universe will be sterile for intelligent beings in a short time.

Could a star be so far away that the light from it is too weak for us to see?

 Easily.

A calculated gamble was taken to use the Hubble Space Telescope — pointing it at a known, very dark area of sky. The total exposure time was of 11 days.

They got this:

That is the Hubble Deep Field and those aren't stars but $3,000$ entire galaxies of stars invisible to the naked eye.

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It takes a few photons of light hitting your retina in a short space of time for you to perceive something is there. While there are a massive number of photons emitted by a star, the number per unit area drops off by the inverse square law.

A certain amount of photons passing through an area at a given distance passes through $2^2=4$ times the area at $2$ times the distance, $3^2=9$ times the area at $3$ times the distance, $4^2=16$ times at $4$ times etc. The number of photons per unit area drops off quickly and so much so that the photons from the galaxies in the Hubble Deep Field arrive far too far apart in time to trigger a human retina and let you know anything is there.

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That image is one twelfth the width of the Moon. Looking away from our galaxy's disc (where there would be significant dust to get in the way) you should expect a similar number of galaxies would be found over most of the sky and doing something similar $3$ years later in the southern sky produced something similar.

But as with the original almost all of the galaxies that are actually in the night sky are invisible to the naked eye.

How big is the observable universe if the distance to the Andromeda Galaxy is one foot?

If the distance from Earth / Milky Way Galaxy to the Andromeda Galaxy were a foot when the real distance is 2.5 million light years, then the 150,000 light year diameter Andromeda Galaxy would be the size of a dime and the Milky Way about 2/3 that size.

There are about 100 mostly small and few big galaxies within 2.5 million light years of us. You can see the Andromeda galaxy in a very dark sky with the naked eye a few times bigger than the moon’s apparent size.

The radius of the observable universe accounting for expansion of space isn’t 13.5 billion light years since the big bang but 46 billion light years. That’s only about 18,400 times the Andromeda distance, so it would be 3.5 miles in radius or 6.8 miles in radius about the size of a city center. That’s a big number but nowhere near infinity at that scale.

The volume of the sphere would be 26 trillion cubic feet, which you could compare with high estimates of the number of galaxies as 20 trillion.

The actual universe is larger than what is observable limited by the amount of space light can travel since the big bang, and no one knows if it is infinite or just really really big.

What implications in the Star Wars universe are actually horrifying?

 I found this meme adequately sums it up:

ABE: Wow. I didn’t expect anyone to take this that seriously…I just thought it was an amusing take, sort of like those “Sum up a movie plot poorly in one sentence” kind of things. (Yes, I know it’s actually two sentences.)

AABE: I would also like to mention that nobody makes that big a deal about the fact that Han Solo was, in fact, a drug smuggler, who was promoted to fricking General. I know “Solo” tries to make it look like the stuff smuggled from “the spice mines of Kessel” was a fuel, but let’s be real: “Spice” is not fuel. And Han wasn’t risking his life for pepper.

What is beyond the observable universe? It is mind boggling to think about infinity.

 The thing is that we can’t observe anything beyond the observable universe, so we don’t know what is there. However, it assumed that there is more of the same. Galaxies and space, possible infinitely more of it. Analyses of the Cosmic Microwave Background (CMB) indicate that the curvature of space is flat (possibly infinite). But within the error bars of the measurement the universe outside our observable part is at least 250 times larger than the observable part.

The farther away we look, the more the light is red shifted, which put requirements on our observing equipment how far we can see. The farther we look, the farther back in time we also see. The James Webb Telescope is an improvement in that respect, since it can in principle see in the infrared 13.6 billion years back in time.

The absolute edge that we can see is the CMB, which is the first visible light and corresponded to a black body radiation of 3000°K when it was emitted. It is now “stretched” to 2.7°K. The matter that emitted it was receding from us very close to the speed of light and from a gravity well, causing the redshift.

Then there is dark energy that speeds up distant matter going away from us, making it disappear over the observable edge. On the other hand, tomorrow we will see a bit more of the CMB light that has been moving in our direction from far away for one day longer (13.8 billion years plus one day).

How large is the universe assuming it expanded at the speed of light since the beginning?

 If nothing can travel faster than light, how is a 13.8-billion-year-old universe 93 billion light-years across?

If the universe expanded at the speed of light in all directions from a starting point, the radius of this cosmic sphere would be precisely 13.8 billion light-years. To find the total size, you simply double the radius to get the diameter, resulting in a universe that is exactly 27.6 billion light-years across.

However, the reality of cosmology is much stranger and more fascinating. The actual observable universe is vastly larger than 27.6 billion light-years across; it spans roughly 93 billion light-years in diameter.

This massive discrepancy exists because the speed of light is only the absolute speed limit for objects traveling through space. It does not dictate the rules for the expansion of space itself. In the earliest moments of the Big Bang, during a phase known as cosmic inflation, the very fabric of spacetime stretched outward exponentially, at a rate vastly faster than the speed of light. Furthermore, space has continued to expand ever since. When astronomers look at the most distant galaxies, they are seeing light that has traveled for over 13 billion years, but in the meantime, the space between those galaxies and Earth has relentlessly continued to stretch.

So, while a universe strictly bound by the speed of light would measure a tidy 27.6 billion light-years from edge to edge, the physics of a stretching spacetime created a much grander and more expansive cosmos.

What is an example of how large the universe is that will truly blow my mind?

 Fairly local examples:

Because it looks pretty in books, the Earth-Moon distance is presented like this:

But the distance indicated shows the Moon should be around 30 Earth diameters away.

That is an actual image taken in the infrared by the Mars Odyssey mission as it was on its way to the Red Planet. If you zoom in you can see the cold (so dark) South Pole of the Earth.

You also see the planets presented like this:

If you did have a view from far enough away to see the orbits of the planets you wouldn't actually be able to see any planets. The distances between them are too large and the planets, relatively speaking, are too small. And even that picture fails to show the scale of the Sun and planets. They are more like this:

And see graphics of the asteroid belt like this…

…and you probably think of this:

In reality, if you were where any particular asteroid was the chances of seeing the next is very close to zero — they are an average of a million kilometres apart.

How does the idea of a cyclical universe, with repeated Big Bangs and Big Crunches, align with current scientific understanding?

 The poetic idea of a universe endlessly dying and being reborn in a fiery "Big Crunch" was killed in the late 1990s. Today, observational cosmology tells a much colder, lonelier story.

The traditional cyclical model relied on a simple premise: if there is enough matter in the universe, the collective gravitational pull will eventually halt the expansion started by the Big Bang. The universe would then collapse back into a searing, infinitely dense singularity, potentially bouncing back out in a new Big Bang.

However, astronomers measuring distant supernovae found that the expansion of the universe is not slowing down; it is accelerating. The driver of this acceleration is an enigmatic force called dark energy, which makes up roughly 68 percent of the universe. Because dark energy actively pushes the fabric of space apart faster than gravity can pull it together, a standard gravitational collapse is currently considered impossible.

Instead of a Big Crunch, the current scientific consensus points toward a "Big Freeze" or "Heat Death." The universe will likely expand forever. Eventually, galaxies will drift so far apart that they become isolated, the last stars will burn out, and all matter will slowly decay into a thin soup of cooling radiation.

Despite this, the concept of a cyclical universe is not entirely dead in modern physics, though the models have evolved past the traditional Big Crunch:

• Conformal Cyclic Cosmology (CCC): Proposed by physicist Roger Penrose, this theory suggests that in the unimaginably distant future, a universe reduced to nothing but massless photons effectively loses the concept of time and physical scale. Mathematically, this vast, empty state is identical to the infinitely small, dense conditions of a new Big Bang.
• Ekpyrotic Models: Rooted in string theory, these models suggest that the visible universe lies on a three-dimensional "brane" that periodically collides with a parallel brane across a higher-dimensional space. Each collision sparks a new Big Bang without requiring the universe itself to contract first.

While these modern cyclic theories are mathematically fascinating, they remain highly speculative. Based on all current physical evidence available up to 2025, the universe is on a one-way trip toward endless, accelerated expansion.

Why do some theories suggest the universe originated from other dimensions, and what do they imply about our understanding of reality?

 The Big Bang might not have been an explosion from nothingness, but a violent collision between two parallel dimensions.

This idea emerges from a mathematical necessity to solve some of the most stubborn paradoxes of our origins. In the standard cosmological model, the universe expands from a singularity—a point of infinite density and temperature where all known laws of physics completely break down. Because singularities are mathematically impossible to describe with current physics, theoretical physicists have sought alternative models to explain how everything began.

One of the most prominent alternatives comes from string theory and its extension, M-theory. These frameworks require the existence of extra spatial dimensions—up to 11 in total—for their mathematics to function. In this view, familiar three-dimensional space is just a single membrane, or "brane," floating within a much larger, higher-dimensional space known as the "bulk."

This leads to the Ekpyrotic model of the universe. This theory suggests that the universe was born from a collision between two parallel branes across a fourth spatial dimension. When these branes smashed together, the immense kinetic energy of the impact was converted into the heat, light, and matter that filled the early universe. This model neatly explains the uniform temperature of the cosmic microwave background without relying on the rapid, unexplained expansion known as cosmic inflation.

If these dimensional theories hold true, they imply a radical shift in how reality is understood:

• A cyclical cosmos: Instead of a universe with a definitive beginning and a cold, expanding end, the brane collision model allows for a cyclic universe. Branes could pull apart, empty out over trillions of years, and eventually crash together again, creating an endless loop of cosmic death and rebirth.
• Leaking gravity: This framework offers an elegant explanation for why gravity is drastically weaker than the other fundamental forces, like electromagnetism. While light and matter are stuck to the three-dimensional brane, gravity might be free to propagate across the extra dimensions, diluting its strength in the observable universe.
• Hidden worlds: Reality would be vastly larger than what telescopes can see. Entire other universes could be hovering mere fractions of a millimeter away in a higher dimension, completely invisible and inaccessible except perhaps through the subtle pull of gravity.

Ultimately, these theories suggest that the observable universe is just a small cross-section of a much deeper, vastly more complex structure.

Why does the universe create objects as extreme as black holes?

 Here main player is gravity. When a massive star(>3 solar masses) collapse due to end of it’s fuel,then outward pressure is lower than the inward gravitational force and it creates a black hole.

According to GR, space time curvature bends heavily in case of a black hole. Thus we see that when gravity wins over expansion force,that creates high density region of mass so that nothing can escape from it nor even light. It is called black hole.

Now,unlike gravity creates star, galaxy, cluster ,it ,creates also black holes. In center of every galaxy there is a supermassive black hole.

The relationship is better described as mutual sculpting — the galaxy feeds the black hole, and the black hole regulates the galaxy. It's a deeply coupled system, not a one-way preservation mechanism(by Cormendy and Ho’s research paper).

Image credit: vox

How does the spin and geometry of the universe tie into the Big Bang and everything we see around us now?

 In 1949, mathematician Kurt Gödel proved that a spinning universe could theoretically allow time travel into the past, but observational data has since revealed a much more structured, non-rotating reality. The spin and geometry of the cosmos are intricately tied to the Big Bang, and they perfectly dictate why the modern universe is organized the exact way it is.

Starting with geometry, the observable universe is astonishingly "flat." In cosmology, a flat universe does not mean space is a two-dimensional plane. Instead, it means that standard Euclidean geometry applies on the largest cosmic scales: parallel lines will never intersect, and the angles of a massive triangle drawn between three distant galaxies will add up to exactly 180 degrees. This flatness is a direct consequence of a brief moment immediately following the Big Bang known as cosmic inflation. During inflation, space expanded exponentially. Just as blowing up a massive balloon makes its curved surface appear flatter to an ant walking across it, this immense expansion stretched the fabric of the universe so intensely that any initial curvature was completely ironed out.

When it comes to spin, modern astrophysics points to a universe that does not rotate at all. Detailed maps of the Cosmic Microwave Background—the lingering thermal radiation from the Big Bang—show no evidence of cosmic rotation. If the universe were spinning, it would inherently possess an axis of rotation, meaning space would have a "preferred" direction. This would violate the cosmological principle, the foundational rule that the universe is isotropic, meaning it looks fundamentally the same in every direction.

These two traits—a flat geometry and a lack of spin—are the reasons the sky looks the way it does today. Because space is flat, light from distant galaxies travels in straight, predictable lines over billions of light-years, allowing for the stable formation of cosmic structures. Because it does not spin, there is no physical center to the universe and no cosmic edge. Galaxies and galaxy clusters are distributed evenly throughout the cosmos, existing in a delicate, unspooled balance rather than swirling around a giant central vortex.

Why does the universe allow black holes to exist at all?

 There exist black holes in the universe because any star which is greater than 3 times solar masses can go into three types objects in their final phases.They may be white dwarf,neutron star or black hole. So forming a black hole is an inevitable phenomenon In our cosmos.

Now turn to the main question, why?

Because black hole regulates star formation in it's host galaxy. There is a huge correlation between black hole and galaxy co-evolurion. So without black hole, galaxy might not be fully structured.

In center of each galaxy ,there is a supermassive black hole. It preserves angular momentum of that system. When there exist too many masses in a small region,they don't let go anything outside nor even light. So these masses creates curve in space-time curvature and gravity is too high.This high density region is known as black hole.It is also a consequence of theory of general relativity by Einstein.

So existence of black hole in space proved by theoretically and experimentally both.

Why do supervoids exist in the universe?

No one knows everything for sure about the dark spaces. We have theories. But you're right, Supervoids exist. They are the cold, empty deserts of the universe.

To understand them, you must look back to the beginning. When the universe was young, matter was never perfectly smooth. Tiny ripples. Existed. Gravity began its long, slow work. It pulled things - the heavy parts close together.

These parts became bright stars, they became galaxies. They formed thick webs. Gravity built these structures, but it stole from the other places. The blank parts grew even emptier - A thing called dark energy joined the long fight.

We don't know exactly when. But it pushes everything apart - stretches the void out wide.

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Gargantuan. Quiet. Deep spaces.

Gravity won the battle in the bright clusters. The dark voids were left behind.

What is the most disturbing fact about the universe?

 The creepiest part of the Universe is that it's mostly patch black empty space.

If the milky way and andromeda galaxies merge it's unlikely any stars will collide because of staggeringly empty space.

If you still can't comprehend how empty space is, here is an example.

Imagine you are traveling in space and a huge cloud of sand is approaching at high speed. Even if your ship gets hit by one grain of sand, it can severely damage your spaceship.

Except that wouldn't happen because each grain of sand on average is TEN KILOMETERS apart from every other grain of sand.

That much empty the space.

What are some mind-blowing facts about the universe?

 

• Our solar system’s biggest mountain is on Mars.

• Space has no atmosphere.
• Sound cannot travel in space as there is no medium

• The nearest star to earth is 4.2 light-years away.

• A spoonful of a neutron star weighs about a billion-ton.

• You can't cry on space, because your tears won't ever fall.

• In 1977 we received a signal from deep space that lasted 72 seconds. We still don't know how or where it came from.

• Saturn's rings are not solid. They are made up of bits of Ice. Dust and Rock.

• A single day in planet Venus is longer than its year.
• Venus takes 243 Earth days to rotate once on its axis. The planet's orbit around the Sun takes 225 Earth days.)

• The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to travel across it.

• Black holes are so dense that even the light cannot escape.