```ts
/**
 * Helper function to validate that the provided temperature is finite and at or above absolute zero.
 * @param temp The temperature in degrees Celsius.
 */
function _validateTemperature(temp: number): void {
  if (typeof temp !== "number" || !Number.isFinite(temp)) {
    throw new Error("temp must be a finite number");
  }
  if (temp < -273.15) {
    throw new Error("temp must be at or above absolute zero");
  }
}

/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  _validateTemperature(celsius);
  return celsius * 1.8 + 32;
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  _validateTemperature(celsius);
  return celsius + 273.15;
}
```