The Universe in Your Hand

1. Our Universe is a Vast, Dynamic Cosmos.

You are about to start a journey through the universe as it is understood by science today.

Beyond our senses. Our everyday perception of the universe is limited. From a beach, we see stars in our galaxy, the Milky Way, but the true scale is vastly larger. The Milky Way is just one island of stars among billions of galaxies.

Cosmic structures. Galaxies group into clusters and superclusters, forming immense filament-like structures across the visible universe. These structures are engaged in a cosmic ballet, moving, merging, and evolving over billions of years.

• Milky Way: Our galaxy, ~300 billion stars.
• Andromeda: Our closest large galactic neighbor, ~1000 billion stars.
• Local Group: Our small cluster of ~54 galaxies.
• Superclusters: Contain thousands of galaxies.

Time scales. Cosmic events unfold over eons, making human existence seem instantaneous. The Sun will die in 5 billion years, and our galaxy will collide with Andromeda in 4 billion years, highlighting the immense time scales of the cosmos.

2. Gravity is the Bending of Spacetime.

Gravity is just geometry.

Beyond force. Isaac Newton described gravity as a force pulling objects together, but Albert Einstein revolutionized this view. Gravity is not a force, but the result of mass and energy bending the fabric of the universe, known as spacetime.

Spacetime curves. Massive objects like planets and stars create curves or slopes in spacetime around them. Objects move along these curves, which we perceive as falling or orbiting.

• Earth's gravity: A steep slope keeping us grounded.
• Sun's gravity: A larger slope keeping planets in orbit.
• Black holes: Create extremely steep slopes.

Universal effect. Everything with mass or energy bends spacetime, including light. This bending explains phenomena like the precise orbit of Mercury and the deflection of starlight around massive objects.

3. Speed Distorts Time and Space (Relativity).

Speed changes everything.

No universal clock. Einstein's theory of special relativity revealed that time is not absolute. A clock moving very fast relative to another will tick slower, a phenomenon called time dilation.

• High-speed travel: A fast-moving traveler ages less than someone stationary.
• GPS satellites: Need relativistic corrections for accurate timekeeping.

Length contraction. Along with time dilation, objects moving at high speeds appear shorter in the direction of motion to a stationary observer. Distances also contract.

Speed limit. Nothing with mass can reach or exceed the speed of light. As an object approaches light speed, its mass increases, requiring infinite energy to accelerate further. Light itself is massless, which is why it travels at the ultimate speed limit.

4. The Quantum World Governs the Very Small.

Those who are not shocked when they first come across quantum theory cannot possibly have understood it.

Beyond intuition. The world of atoms and subatomic particles behaves in ways that defy our everyday experience and intuition. Particles do not act like tiny balls but exhibit wave-like properties.

Uncertainty. At the quantum level, it's impossible to know certain pairs of properties simultaneously with perfect accuracy. For example, the more precisely you know a particle's position, the less precisely you can know its momentum (velocity and direction).

Quantum jumps. Particles can instantaneously move from one energy level to another or even tunnel through barriers without physically crossing the space in between. This "quantum tunnelling" is a real phenomenon used in technologies like scanning tunnelling microscopes.

5. Matter and Forces Emerge from Quantum Fields.

The field you’ve just been introduced to, this invisible mist that permeates the entire universe and becomes somewhat more active near and in between charged objects, is much more than that.

Universe of fields. Everything we see and interact with is understood to arise from fundamental quantum fields that permeate all of spacetime. Particles are not tiny solid balls but excitations or ripples in these fields.

Fundamental forces. Four fundamental forces govern interactions in the universe:

• Electromagnetic: Governs light and charged particles (electrons, quarks). Carried by photons.
• Strong Nuclear: Binds quarks in protons/neutrons and nuclei together. Carried by gluons and mesons.
• Weak Nuclear: Responsible for radioactive decay. Carried by W and Z bosons.
• Gravity: Governs attraction between mass/energy. Hypothetically carried by gravitons.

Particle interactions. Particles interact by exchanging force-carrying particles (virtual particles) that pop in and out of existence from the field's vacuum. Electrons repel by exchanging virtual photons; quarks are bound by exchanging gluons.

6. The Universe Began Hot and is Expanding (Big Bang).

It means that space stretches, and therefore grows, on its own, in between the galaxies.

Cosmic history. Observations show that distant galaxies are moving away from us, and the further they are, the faster they recede. This indicates that the universe itself is expanding.

Rewinding time. If the universe is expanding now, it must have been smaller and denser in the past. Extrapolating backwards leads to the idea of a hot, dense state from which the universe emerged, known as the Big Bang.

Evidence. Key evidence for the Big Bang includes:

• Cosmic Microwave Background (CMB): Leftover heat radiation from the early universe, detected everywhere at a uniform temperature.
• Abundance of light elements: The proportions of hydrogen and helium match predictions for a hot early universe.

7. Most of the Universe Remains a Dark Mystery.

Everything you’ve seen so far throughout your journeys corresponds to only 4.6 per cent of the total content of our universe.

Invisible majority. Despite our understanding of ordinary matter (protons, neutrons, electrons), it constitutes less than 5% of the universe's total energy content. The vast majority is made of unknown substances.

Dark Matter. Observations of galaxy rotation and galaxy cluster movements show that there is much more gravitational pull than visible matter can account for. This unseen mass is called dark matter.

• It interacts gravitationally but not electromagnetically (doesn't emit or absorb light).
• It is estimated to be about 5 times more abundant than ordinary matter.

Dark Energy. Observations of distant supernovae show that the universe's expansion is accelerating, not slowing down as expected from gravity. This acceleration is attributed to a mysterious force or energy called dark energy.

• It acts as an anti-gravitational force, pushing spacetime apart.
• It makes up about 72% of the universe's total energy content.

8. Gravity and Quantum Physics Clash at Extremes.

In other words, it means that general relativity theory and quantum field theory do not use the same notions of space and time.

Incompatible theories. Our two most successful theories, general relativity (for the very large, gravity) and quantum field theory (for the very small, particles/forces), are fundamentally incompatible. They use different concepts of space and time.

Breakdown points. Both theories predict their own failure at extreme conditions:

• General Relativity: Breaks down at singularities (infinite density/curvature) found at the center of black holes and potentially at the universe's origin.
• Quantum Field Theory: Predicts infinite energy density in the vacuum unless mathematical "renormalization" tricks are used, which don't work when gravity is included quantumly.

Need for unification. A complete understanding of the universe requires a theory that unifies gravity and quantum mechanics (quantum gravity). This theory is needed to describe phenomena where both extremes meet, such as the very early universe and the interiors of black holes.

9. Black Holes are Windows to Quantum Gravity.

Black-hole radiation is the only proof so far that our theories might reflect nature in that respect.

Extreme environments. Black holes are formed from the collapse of massive stars, creating regions of spacetime curvature so extreme that nothing, not even light, can escape beyond a boundary called the event horizon.

Hawking Radiation. Stephen Hawking discovered that black holes are not entirely black but emit radiation due to quantum effects near the event horizon. This "Hawking radiation" causes black holes to slowly lose mass and evaporate over time.

Information paradox. Hawking radiation appears to be independent of what falls into the black hole, suggesting that information about the swallowed matter is lost. This contradicts a fundamental principle of quantum mechanics, creating the black hole information paradox, a major mystery in physics.

10. The Vacuum is a Sea of Quantum Activity.

I will say it once more: there is no such thing as emptiness in this universe of ours.

Beyond void. What appears to be empty space is, according to quantum field theory, a dynamic realm filled with quantum fields. These fields are constantly fluctuating.

Virtual particles. The vacuum is not empty but is teeming with "virtual" particle-antiparticle pairs that spontaneously pop into existence and disappear almost immediately, borrowing energy from the field.

Casimir effect. The reality of vacuum fluctuations is supported by the Casimir effect, where two closely spaced metal plates experience a force due to differences in vacuum fluctuations inside and outside the plates. This demonstrates that the vacuum has physical effects.

11. Fundamental Forces May Be Unified.

This was an extraordinary breakthrough in its own right (hence the Nobel Prize), but it also paved the way for something much, much bigger: the tantalizing prospect of unifying all the known forces of nature into just one force (and therefore one theory).

Quest for unity. Physicists seek a single theory that describes all fundamental forces as different aspects of one underlying force or field. This quest is driven by the elegance and simplicity such a unification would bring.

Electroweak unification. The electromagnetic and weak nuclear forces have been successfully unified into a single electroweak force, which existed at very high energies in the early universe. This was confirmed by the discovery of the W and Z bosons.

Grand Unified Theories (GUTs). The next step is to unify the electroweak force with the strong nuclear force into a Grand Unified Theory. This would require even higher energies, potentially found only in the very early universe.

12. Beyond the Known: Inflation and Multiverses.

Eternal inflation may (and should) sound completely crazy to you (it does to me, but I like it), and yet, compared to the strings you are about to encounter, well, nothing will ever sound sane ever again, really . . .

Cosmic Inflation. To explain the uniformity of the cosmic microwave background and the origin of cosmic structures, the theory of inflation proposes a period of extremely rapid, faster-than-light expansion in the universe's earliest moments, driven by a hypothetical inflaton field.

Eternal Inflation. Some models suggest that the inflaton field never fully decays, leading to a process of "eternal inflation" where new "bubble universes" are constantly popping into existence, creating a vast multiverse.

String Theory. One leading candidate for a theory of quantum gravity and a potential theory of Everything is string theory.

• Proposes fundamental particles are not points but tiny vibrating strings.
• Requires extra spatial dimensions beyond the familiar three.
• Different vibration modes of strings correspond to different particles and forces, including gravity.
• Suggests our universe might be a "brane" (membrane) within a higher-dimensional space, potentially alongside other branes (universes).

Last updated: June 2, 2025