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Related Concept Videos

Hess's Law03:40

Hess's Law

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.
Henderson-Hasselbalch Equation02:48

Henderson-Hasselbalch Equation

The ionization-constant expression for a solution of a weak acid can be written as:
The Number e as a Limit01:29

The Number e as a Limit

The number e is a fundamental constant in calculus, playing a central role in describing continuous change, particularly exponential growth. It is most naturally defined through its relationship with the natural logarithm, which is the inverse of the exponential function with base e. This relationship allows e to be characterized using basic principles of differentiation rather than as an arbitrary numerical constant.A key property of the natural logarithm function, ln x, is that its derivative...
Complex Numbers01:29

Complex Numbers

The real number system cannot represent the square root of a negative number, which restricts solutions for certain equations, such as quadratics with negative discriminants. To address this, the complex number system was developed, introducing the imaginary unit i, where i = √(-1). This extension allows for the representation of all roots, including those involving negative radicands.A complex number is written in the form x + yi, where x and y are real numbers. Here, x represents the real...
Fundamental Theorem of Algebra01:30

Fundamental Theorem of Algebra

The Fundamental Theorem of Algebra is central to the study of polynomial equations, asserting that every non-constant polynomial with complex coefficients has at least one complex zero. This means that a polynomial of degree n ≥ 1, written as:  with an ≠ 0, has at least one solution in the complex number system. Since the set of real numbers is a subset of complex numbers, this theorem applies equally to polynomials with real coefficients.Building on this result, the Complete Factorization...
Complex Zeros01:29

Complex Zeros

Complex zeros are the solutions to polynomial equations that include imaginary numbers, specifically, numbers of the form a + bi, where a and b are real numbers and i is the imaginary unit defined by i2=-1. These zeros satisfy the equation P(x) = 0, where P(x) is a polynomial with real or complex coefficients. Since the complex number system includes all real numbers, it provides a complete framework for analyzing all possible roots of a polynomial.Every polynomial of degree n≥1 can be...

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Galaxies, human eyes, and artificial neural networks.

Science (New York, N.Y.)·1995
Same author

Mapping the universe.

Science (New York, N.Y.)·1989
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Updated: Jul 8, 2026

Blast Quantification Using Hopkinson Pressure Bars
09:41

Blast Quantification Using Hopkinson Pressure Bars

Published on: July 5, 2016

The hubble constant.

J P Huchra

    Science (New York, N.Y.)
    |April 17, 1992
    PubMed
    Summary

    The Hubble constant measures the universe's expansion rate. Current measurements vary significantly, creating inconsistencies with cosmological models and theories.

    Area of Science:

    • Cosmology
    • Astrophysics

    Background:

    • The Hubble constant (H0) quantifies the universe's expansion rate, crucial for determining cosmic scale and age.
    • Accurate H0 measurement is vital for validating cosmological models and understanding cosmic evolution.

    Purpose of the Study:

    • To highlight the ongoing challenges and discrepancies in determining the Hubble constant.
    • To discuss the implications of differing H0 values on cosmological theories.

    Main Methods:

    • Measurements of galaxy recession velocity and distance are used to determine H0.
    • Development of new techniques for precise galaxy distance measurements is ongoing.

    Main Results:

    • Current H0 determinations exhibit a wide range, nearly a factor of two.

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    A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference
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  • Local measurements often yield higher H0 values, conflicting with some theoretical predictions.

    • Conclusions:

    • Persistent calibration uncertainties and systematic errors hinder a definitive H0 value.
    • The discrepancy between local and global H0 measurements poses a significant challenge to current cosmological understanding and theories of structure formation and stellar evolution.